NCERT Solutions for Class 11 Chemistry Chapter 1 Some Basic Concepts of Chemistry

NCERT Solutions for Class 11 Chemistry Chapter 1- (Exercise Questions)

Question 1.1 Calculate the molar mass of the following:
(i) H 2 O

Answer : The molar mass of Water $(H_{2}O)$ is:

$= 2\times Atomic\ mass\ of\ hydrogen\ + \ Atomic\ mass\ of\ oxygen$

$= 2\times(1.008\ u) + 16.00\ u$

$= 18.02\ u$

Question 1.1 Calculate the molar mass of the following: (ii) CO2

Answer : The molar mass of Carbon dioxide $CO_{2}$ is:

$= 1\times Atomic\ mass\ of\ carbon\ + \ 2\times Atomic\ mass\ of\ oxygen$

$= 1\times(12.011\ u) + 2\times (16.00\ u)$

$= 44.011\ u$

Question 1.1 Calculate the molar mass of the following:
(iii) CH 4

Answer : The molar mass of Methane $CH_{4}$ is:

$= 1\times Atomic\ mass\ of\ carbon\ + \ 4\times Atomic\ mass\ of\ hydrogen$

$= 1\times(12.011\ u) + 4\times (1.008\ u)$

$= 16.043\ u$

Question 1.2 Calculate the mass per cent of different elements present in sodium sulphate (Na 2 SO 4 ).

Answer : The different elements present in sodium sulphate are:

Sodium (Na), Sulphur (S), and Oxygen (O).

The molar mass of sodium sulphate $Na_{2}SO_{4}$ ; $= (2\times Atomic\ mass\ of\ Na) + (1\times Atomic\ mass\ of\ S) + (4\times Atomic\ mass\ of\ O)$

$= 2\times (23.0\ u) + (32.066\ u) + 4\times (16.00\ u)$

$= 142.066\ u$

So, the mass percentage of an element in a compound is given by,

$= \frac{Mass\ of\ element\ in\ the\ compound}{Molar\ mass\ of\ the\ compound} \times 100$

Therefore,

The mass per cent of Sodium (Na):

$=\frac{46.0\ u}{142.066\ u} \times 100$

$=32.379\%$

$\approx 32.4\%$

The mass per cent of Sulphur (S):

$=\frac{32.066\ u}{142.066\ u} \times 100$

$= 22.57 \%$

$\approx 22.6\%$

The mass per cent of Oxygen (O):

$=\frac{64.0\ u}{142.066\ u} \times 100$

$= 45.049 \%$

$\approx 45.0\%$

Question 1.3 Determine the empirical formula of an oxide of iron, which has 69.9% iron and 30.1% dioxygen by mass.

Answer : Given there is an oxide of iron which has $69.9\%$ iron and $30.1\%$ dioxygen by mass:

Relative moles of iron in iron oxide:

$= \frac{\%\ of\ iron\ by\ mass}{Atomic\ mass\ of\ iron}$

$= \frac{69.9}{55.85} = 1.25$

Relative moles of oxygen in iron oxide:

$= \frac{\%\ of\ oxygen\ by\ mass}{Atomic\ mass\ of\ oxygen}$

$= \frac{30.1}{16.00} = 1.88$

The simplest molar ratio of iron to oxygen:

$\Rightarrow 1.25:1.88$ $\Rightarrow 1:1.5$ $\Rightarrow 2:3$

Therefore, the empirical formula of the iron oxide is $Fe_{2}O_{3}$ .

Question 1.4 Calculate the amount of carbon dioxide that could be produced when

(i) 1 mole of carbon is burnt in the air.

Answer : When carbon is burnt in the air:

The chemical equation for this reaction is:

$C_{(s)}+O_{2(g)} \rightarrow CO_{2(g)}$

Here 1 mole of carbon (solid) weighing 12g is burnt in 1 mole of Dioxygen (gas) weighing 32g to produce 1 mole of carbon dioxide (gas) weighing 44g.

Question 1.4 Calculate the amount of carbon dioxide that could be produced when

(ii) 1 mole of carbon is burnt in 16 g of dioxygen.

Answer : When carbon is burnt in 16 g of dioxygen:

The chemical equation for this reaction is:

$C_{(s)}+O_{2(g)} \rightarrow CO_{2(g)}$

Here, oxygen acts as a limiting reagent as only 16g of dioxygen is available.

Hence, it will react with 0.5 mole of carbon to give 22g of carbon dioxide.

Question 1.4 Calculate the amount of carbon dioxide that could be produced when

(iii) 2 moles of carbon are burnt in 16 g of dioxygen.

Answer : When 2 moles of carbon is burnt in 16 g of dioxygen:

The chemical equation for this reaction is:

$C_{(s)}+O_{2(g)} \rightarrow CO_{2(g)}$

Here, oxygen acts as a limiting reagent as only 16g of dioxygen is available.

Thus, 16g of dioxygen will react with 0.5 mole of carbon to give 22g of carbon dioxide.

Question 1.5 Calculate the mass of sodium acetate (CH 3 COONa) required to make 500 mL of 0.375 molar aqueous solution. Molar mass of sodium acetate is 82.0245 g mol –1 .

Answer : $0.375$ molar aqueous solution would contain $0.375$ moles of $CH_{3}COONa$ dissolved in $1000mL$ of solvent.

So, we have to calculate for $500mL$ solution of $CH_{3}COONa$ .

Therefore the number of moles of sodium acetate in $500mL$ will be:

$= \frac{0.375}{1000}\times 500$

$=0.1875\ mole$

Given Molar mass of sodium acetate: $82.0245\ g\ mol^{-1}$

So, the required mass of sodium acetate $= (82.0245\ g\ mol^{-1})\times (0.1875\ mole)$

$= 15.38\ grams$

Question 1.6 Calculate the concentration of nitric acid in moles per litre in a sample which has a density, 1.41 g mL - 1 and the mass percent of nitric acid in it being 69%.

Answer : Given the mass percentage of nitric acid is $69\%$ .

That means 69 grams of nitric acid are present in 100 grams of nitric acid solution.

The molar mass of nitric acid $HNO_{3}$ is $1+14+ 3\times (16) = 63g\ mol^{-1}$ .

So, the number of moles in 69g of Nitric acid:

$= \frac{69}{63}\ moles = 1.095\ moles$

and volume of 100g of the nitric acid solution:

$=\frac{100}{1.41}mL = 70.92mL = 0.07092\ L$

Therefore, the concentration of Nitric acid in moles per litre is:

$= \frac{1.095}{0.07092} = 15.44\ M$

Question 1.7 How much copper can be obtained from 100 g of copper sulphate (CuSO 4 )?

Answer : Given that 100g of Copper sulphate $CuSO_{4}$ ;

1 mole of $CuSO_{4}$ contains 1 mole of copper.

Molar Mass of Copper sulphate is:

$= 63.5 + 32.00 + 4\times 16.00 = 159.5g$

Now, $159.5g$ of copper sulphate contains $63.5g$ of copper.

So, $100g$ of copper sulphate will contain copper content:

$=\frac{63.5}{159.5}\times 100 = 39.81g$

Question 1.8 Determine the molecular formula of an oxide of iron, in which the mass percent of iron and oxygen are 69.9 and 30.1, respectively. Given that the molar mass of the oxide is $159.69 g\ mol^{-1}$ .

Answer :Given that the mass percentage of iron is $69.9\%$ and the mass percentage of oxygen is $30.1\%$

The atomic mass of iron $= 55.85\ u$ .

The atomic mass of oxygen $=16.00\ u$ .

So, the relative moles of iron in iron oxide will be:

$=\frac{mass\ of\ iron\ by\ mass}{Atomic\ mass\ of\ iron}$

$= \frac{69.9}{55.85} = 1.25$

And relative moles of oxygen in iron oxide will be:

$=\frac{mass\ of\ oxygen \ by\ mass}{Atomic\ mass\ of\ oxygen}$

$= \frac{30.01}{16.00} = 1.88$

Hence the simplest molar ratio:

$=\frac{1.25}{1.88}$ or $\Rightarrow 1:1.5 = 2:3$

Therefore, the empirical formula of iron oxide will be $Fe_{2}O_{3}$ .

Now, calculating the molar mass of $Fe_{2}O_{3}$ :

$=(2\times 55.85) + (3\times16.00) = 159.7g\ mol^{-1}$ .

Hence it is matching with the given molar mass of the oxide.

Question 1.9 Calculate the atomic mass (average) of chlorine using the following data:

% Natural Abundance Molar Mass
³⁵Cl 75.77 34.9689
³⁷Cl 24.23 36.9659

Answer: To calculate the average atomic mass of chlorine:

Given the fractional natural abundance of $_{}^{35}\textrm{Cl}$ with $34.9689\ u$ molar mass is $75.77\%$ and that of $_{}^{37}\textrm{Cl}$ with $36.9659\ u$ molar mass is $24.23\%$ .

Therefore we have,

Average Atomic mass of Chlorine:

$= \left ( 0.7577\times34.9689\ u \right ) + \left ( 0.2423\times36.9659\ u \right )$

$= 26.4959\ u + 8.9568\ u = 35.4527\ u$

Question 1.10 In three moles of ethane (C 2 H 6 ), calculate the following:

(i) Number of moles of carbon atoms.

Answer : Given there are three moles of Ethane $C_{2}H_{6}$ ;

So, 1 mole of $C_{2}H_{6}$ contains 2 moles of carbon atoms.

Therefore, 3 moles of $C_{2}H_{6}$ contains 6 moles of carbon atoms.

Question 1.10 In three moles of ethane (C 2 H 6 ), calculate the following:

(ii) Number of moles of hydrogen atoms.

Answer : Given there are three moles of Ethane $C_{2}H_{6}$ ;

So, 1 mole of $C_{2}H_{6}$ contains 6 moles of hydrogen atoms.

Therefore, 3 moles of $C_{2}H_{6}$ contains 18 moles of hydrogen atoms.

Question 1.10 In three moles of ethane (C 2 H 6 ), calculate the following:

(iii) Number of molecules of ethane.

Answer : Given there are three moles of Ethane $C_{2}H_{6}$ ;

So, 1 mole of $C_{2}H_{6}$ contains $6.02\times 10^{23}$ molecules of ethane.

Therefore, 3 moles of $C_{2}H_{6}$ contains $3\times6.02\times 10^{23} = 18.06\times10^{23}$ molecules of ethane.

Question 1.11 What is the concentration of sugar (C 12 H 22 O 11 ) in mol L –1 if its 20 g are dissolved in enough water to make a final volume up to 2L?

Answer : The molar mass of sugar $C_{12}H_{22}O_{11}$ is:

$= (12\times12)+ (1\times 22)+(11\times16) = 342g\ mol^{-1}$ .

The number of moles of sugar in 20g of sugar will be:

$= \frac{20}{342} = 0.0585\ mole$

and given the volume of the solution after dissolving enough water is 2L .

$Molar\ concentration = \frac{Moles\ of\ solute}{Volume\ of\ solution\ in\ 1L}$

$= \frac{0.0585mol}{2L} = 0.0293mol\ L^{-1} = 0.0293\ M$

Question 1.12 If the density of methanol is 0.793 kg L –1 , what is its volume needed for making 2.5 L of its 0.25 M solution?

Answer :Given that the density of methanol $CH_{3}OH$ is $0.793 Kg\ L^{-1}$ .

So, the number of moles present in the methanol per litre will be or the Molarity of the solution will be :

$= \frac{0.793Kg\ L^{-1}}{0.032Kg\ mol^{-1}} = 24.78\ mol\ L^{-1}$

Now, to make 2.5L of its 0.25M solution:

We will apply the formula: $M_{1}V_{1} (for\ given\ solution) = M_{2}V_{2} (for\ solution\ to\ be\ prepared)$

$24.78\times V_{1} =0.25\times 2.5L$

$V_{1} = 0.02522L\ or\ 25.22\ mL$ .

Hence $25.22\ mL$ volume will be required for making 2.5L of methanol 0.25M solution.

Question 1.13 Pressure is determined as force per unit area of the surface. The SI unit of pressure, pascal is as shown below:
1 Pa = 1 Nm –2
If mass of air at sea level is 1034 g cm –2 , calculate the pressure in pascal.

Answer :The pressure is as given is force per unit area of the surface.

$Pressure = \frac{Force}{Area}$

Given to calculate the pressure exerted by the air on sea water, if the mass of air at sea level is $1034\ g\ cm^{-2}$ .

The force with which the air is exerting on the surface is:

$= \frac{1034g\times9.8ms^{-2}}{cm^2}\times\frac{1kg}{1000g}\times\frac{100cm}{1m} = 1.01332\times10^5\ N$

Now,

as $1 Pascal = 1N\ m^{-2}$

Therefore, $1.01332 \times 10^5 \mathrm{~N}^{-2}=1.01332 \times 10^5 \mathrm{~Pa}$

Question 1.14 What is the SI unit of mass? How is it defined?

Answer : The SI unit of mass is Kilogram (Kg).

It is defined as the mass of platinum-iridium (Pt-Ir) cylinder that is stored in an air-tight jar at the International Bureau of Weigh and Measures in France.

Question 1.15 Match the following prefixes with their multiples:

Prefixes Multiples

(i) micro 10 6
(ii) deca 10 9
(iii) mega 10 –6
(iv) giga 10 –15
(v) femto 10

Answer : Matched items are given in below table:

Question 1.16 What do you mean by significant figures?

Answer : Significant figures are meaningful digits that are known with certainty including the last digit whose value is uncertain.

For example: if we write a result as 56.923 Kg, we say the 56.92 is certain and 3 is uncertain and the uncertainty would be $\pm 1$ in the last digit. Here we also include the last uncertain digit in the significant figures.

Question 1.17 A sample of drinking water was found to be severely contaminated with chloroform, CHCl 3 , supposed to be carcinogenic in nature. The level of contamination was 15 ppm (by mass).

(i) Express this in per cent by mass.

Answer :1ppm means 1 parts in million $(10^6)$ parts.

So, in percentage by mass:

We have,

$Percentage\ by\ mass = \frac{15}{10^6}\times 100 = 15\times10^{-4}$

$= 1.5\times10^{-3}\%$

Question 1.17 A sample of drinking water was found to be severely contaminated with chloroform, CHCl 3 , supposed to be carcinogenic in nature. The level of contamination was 15 ppm (by mass).

(ii) Determine the molality of chloroform in the water sample .

Answer :The molarity of chloroform sample in water will be:

$= \frac{number\ of\ moles\ present}{Volume\ of\ solution\ in\ L}$

The molar mass of Chloroform $CHCl_{3}$ :

$= 12+1+(3\times35.5) = 118.5g\ mol^{-1}$

So we have calculated in the previous part that percentage by mass of chloroform is $1.5\times10^{-3}$ .

Hence in 100g sample, there will be $1.5\times10^{-3}g$ of chloroform.

Therefore, 1000g (1Kg) of the sample will contain choroform $= 1.5\times10^{-2}g$ .

$= \frac{1.5\times10^{-2}}{118.65\ mol} = 1.266\times 10^{-4}\ mol$

Therefore, the molarity of chloroform in the water sample is $1.266\times 10^{-4}\ M$ .

Question 1.18 Express the following in the scientific notation:

(i) 0.0048

Answer : The scientific notation of $0.0048$ will be $4.8\times 10^{-3}$ .

Question 1.18 Express the following in the scientific notation:

(ii) 234,000

Answer :The scientific notation of $234,000$ will be $2.34\times10^5$ .

Question 1.18 Express the following in the scientific notation:

(iii) 8008

Answer : The scientific notation of $8008$ will be $8.008\times 10^3$ .

Question 1.18 Express the following in the scientific notation:

(iv) 500.0

Answer : The scientific notation of $500.0$ will be $5.000\times10^2$ .

Question 1.18 Express the following in the scientific notation:

(v) 6.0012

Answer :

The scientific notation of $6.0012$ will be $6.0012\times10^0$

Question 1.19 How many significant figures are present in the following?

(i) 0.0025

Answer : There are 2 significant digits because all non-zero digits are in a number are significant and the zeros written to the left of the first non-zero digit in a number are non-significant.

Question 1.19 How many significant figures are present in the following?

(ii) 208

Answer :There are 3 significant digits because all non-zero digits are in a number are significant and the zeros between two non-zero digits are always significant.

Question 1.19 How many significant figures are present in the following?

(iii) 5005

Answer : There are 4 significant digits because all non-zero digits are in a number are significant and the zeros between two non-zero digits are always significant.

Question 1.19 How many significant figures are present in the following?

(iv) 126,000

Answer : There are 3 significant digits because all non-zero digits are in a number are significant and the terminal zeros are not significant if there is no decimal point.

Question 1.19 How many significant figures are present in the following?

(v) 500.0

Answer : There are 4 significant digits because the zeros written to the left of the first non-zero digit in a number are non-significant and all zeros placed to the right of a decimal point in a number are significant.

Question 1.19 How many significant figures are present in the following?

(vi) 2.0034

Answer :There are 5 significant digits because all zeros placed to the right of a decimal point in a number are significant.

Question 1.20 Round up the following upto three significant figures:

(i) 34.216

Answer : After round upto three significant figures:

Answer - $34.2$

Question 1.20 Round up the following upto three significant figures:

(ii) 10.4107

Answer : After round upto three significant figures:

Answer - $10.4$ .

Question 1.20 Round up the following upto three significant figures:

(iii) 0.04597

Answer : After round up to three significant figures:

Here the rightmost digit to be removed is more than 5 i.e., 7, then the preceding number is increased by one. So we get $0.0460$ .

Answer - $0.0460$

Question 1.20 Round up the following upto three significant figures:

(iv) 2808

Answer : After round upto three significant figures:

Here the rightmost digit to be removed is more than 5 i.e., 8, then the preceding number is increased by one. So we get $2810$ .

Answer - $2810$

Question 1.21 The following data are obtained when dinitrogen and dioxygen react together to form different compounds:
Mass of dinitrogen Mass of dioxygen
(i) 14 g 16 g
(ii) 14 g 32 g
(iii) 28 g 32 g
(iv) 28 g 80 g
(a) Which law of chemical combination is obeyed by the above experimental data? Give its statement.

Answer : Here if we fix the mass of dinitrogen at 14g, then the masses of dioxygen that will combine with the fixed mass of dinitrogen are 16g, 32g, 32g, and 80g.

The masses of dioxygen bear a whole number ratio of 1:2:2:5.

Hence, the given experimental data obeys the Law of Multiple Proportionals .

The law given by Dalton states that "if two elements can combine to form more than one compound, the masses of one element that combine with a fixed mass of the other element, are in the ratio of small whole numbers".

Question 1.21 The following data are obtained when dinitrogen and dioxygen react together to form different compounds:
Mass of dinitrogen Mass of dioxygen
(i) 14 g 16 g
(ii) 14 g 32 g
(iii) 28 g 32 g
(iv) 28 g 80 g

(b) Fill in the blanks in the following conversions:
(i) 1 km = ...................... mm = ...................... pm
(ii) 1 mg = ...................... kg = ...................... ng
(iii) 1 mL = ...................... L = ...................... dm 3

Answer : (i) As we know in $1km = 1000m$ or $1m = 1000mm$ . or $1 km = 1000\times1000 mm =10^6 mm$

And $1pm = 1\times10^{-12}m$ or $1km = 10^{15} pm$

Therefore we have,

$1 km = 10^6 mm =10^{15} pm$

(ii) As we know in $1kg = 1000g$ or $1g = 1000mg$ . or $1 kg = 1000\times1000 mg$ or $1mg = \frac{1}{10^6} kg=10^{-6}kg$

And $1 ng = 10^{-9}g$ or $1ng = 10^{-9}\times1000mg$ or $1ng = 10^{-6}mg$

or $1 mg =10^6ng$ .

Therefore we have,

$1 mg = 10^{-6} kg =10^{6} ng$ .

(iii) As we know in $1L= 1000mL$ or $1mL =\frac{1}{1000}L = 10^{-3}L$

or $1dm = 0.1 m \Rightarrow 1dm = 10cm$ or $1cm = 0.1dm$

And $1 m L=1 \mathrm{~cm}^3$ or we can write it as: $1mL = \frac{1dm}{10cm}\times\frac{1dm}{10cm}\times\frac{1dm}{10cm}$

And $1mL = 10^{-3}dm^3$

Therefore we have,

$1mL = 10^{-3}L =10^{-3}dm^3$

Question 1.22 If the speed of light is $3.0 \times 10^8 m s^{-1}$ , calculate the distance covered by light in 2.00 ns.

Answer :

Given the speed of light to be $3.0 \times 10^8 m s^{-1}$ , so the distance covered by light in 2.00 ns. will be:

$Distance = Speed\times Time$

$Distance = (3.0\times10^{8}ms^{-1})\times(2.00\times10^{-9}sec) = 6.00\times10^{-1}m = 0.600m$

Therefore, the light will travel 0.600 metres in 2 nano seconds.

Question 1.23 In a reaction
$A + B_2 \rightarrow AB_2$
Identify the limiting reagent, if any, in the following reaction mixtures.

(i) 300 atoms of A + 200 molecules of B

Answer : Here according to the reaction 1 atom of A reacts with 1 molecule of B.

Therefore, 200 molecules of atoms of B will react with 200 atoms of A, thereby left with 100 atoms of A unreacted.

Hence, B is the limiting reagent in this reaction.

Question 1.23 In a reaction
$A + B_2 \rightarrow AB_2$
Identify the limiting reagent, if any, in the following reaction mixtures.

(ii) 2 mol A + 3 mol B

Answer :

Here according to the reaction 1 atom of A reacts with 1 molecule of B.

Therefore, 2 mol of A atoms will react with only 2 mol of B molecules, thereby left with 1 mole of B unreacted.

Hence, A is the limiting reagent in this reaction.

Question 1.23 In a reaction
$A + B_2 \rightarrow AB_2$
Identify the limiting reagent, if any, in the following reaction mixtures.

(iii) 100 atoms of A + 100 molecules of B

Answer : Here, according to the reaction 1 atom of A reacts with 1 molecule of B.

Therefore, all 100 atoms of A will react with 100 molecules of B, so the reaction is stoichiometric and there is no limiting reagent .

Hence, there is no limiting reagent in this reaction.

Question 1.23 In a reaction
$A + B_2 \rightarrow AB_2$
Identify the limiting reagent, if any, in the following reaction mixtures.

(iv) 5 mol A + 2.5 mol B

Answer :Here, according to the reaction 1 atom of A reacts with 1 molecule of B.

Therefore, 2.5 moles of B molecules will react with only 2.5 moles of A atoms, thereby left with 2.5 moles of A unreacted.

Hence, B limiting reagent in this reaction.

Question 1.23 In a reaction
$A + B_2 \rightarrow AB_2$
Identify the limiting reagent, if any, in the following reaction mixtures.

(v) 2.5 mol A + 5 mol B

Answer : Here according to the reaction 1 atom of A reacts with 1 molecule of B.

Therefore, 2.5 moles of A atoms will react with only 2.5 moles of B molecules, thereby left with 2.5 moles of B unreacted.

Hence, A limiting reagent in this reaction.

Question 1.24 Dinitrogen and dihydrogen react with each other to produce ammonia according to the following chemical equation:
$N_2\ (g) + H_2\ (g) \rightarrow 2NH_3\ (g)$

(i) Calculate the mass of ammonia produced if 2.00 × 10 3 g dinitrogen reacts with 1.00 ×10 3 g of dihydrogen.

Answer : Given that if $2.00\times10^3g$ of dinitrogen reacts with $1.00\times10^3g$ of dihydrogen.

From the reaction we have:

1 mole of dinitrogen weighing 28g reacts with 3 moles of dihydrogen weighing 6g to give 2 moles of ammonia weighing 34g.

Therefore, $2.00\times10^3g$ of $N_{2}$ will react with $H_{2}$ $= \frac{6}{28}\times200g = 428.6g$ .

Thus, here $N_{2}$ is the limiting reagent while $H_{2}$ is in excess.

So, $28g$ of $N_{2}$ produces $34g$ of $NH_{3}$ .

Therefore, $2.00\times10^3g$ of $N_{2}$ will produce $= \frac{34}{28}\times 2000g = 2428.57g$ of $NH_{3}$ .

Question 1.24 Dinitrogen and dihydrogen react with each other to produce ammonia according to the following chemical equation:
$N_2\ (g) + H_2\ (g) \rightarrow 2NH_3\ (g)$

(ii) Will any of the two reactants remain unreacted?

Answer : As from the previous part we have:

$N_{2}$ is the limiting reagent and $H_{2}$ is the excess reagent.

Hence, $H_{2}$ will remain unreacted.

Question 1.24 Dinitrogen and dihydrogen react with each other to produce ammonia according to the following chemical equation:
$N_2\ (g) + H_2\ (g) \rightarrow 2NH_3\ (g)$

(iii) If yes, which one and what would be its mass?

Answer : Yes , $H_{2}$ will remain unreacted.

And the mass of dihydrogen left unreacted will be $=1000g-428.6g = 571.4g$ .

Question 1.25 How are 0.50 mol Na 2 CO 3 and 0.50 M Na 2 CO 3 different?

Answer : Calculating the molar mass of $Na_{2}CO_{3}$ :

$= (2\times23)+12.00+(3\times16) = 106g\ mol^{-1}$

Therefore,

$0.50\ mol$ of $Na_{2}CO_{3}$ means:

$0.50\times106g = 53g$

whereas,

$0.50 M$ $Na_{2}CO_{3}$ means:

$0.50\ mol$ of $Na_{2}CO_{3}$ or $53g$ of $Na_{2}CO_{3}$ are present in 1litre of the solution.

Question 1.26 If 10 volumes of dihydrogen gas reacts with five volumes of dioxygen gas, how many volumes of water vapour would be produced?

Answer : For the given situation we have the reaction:

$2H_{2}(g)+O_{2}(g)\rightarrow2H_{2}O(g)$

Here, 2 volumes of dihydrogen gas react with 1 volume of dioxygen to produce 2 volumes of water vapour.

So, if 10 volumes of dihydrogen gas react with 5 volume of dioxygen then it will produce ( $2\times5 = 10$ volumes of water vapour.

Question 1.27 Convert the following into basic units:

(i) 28.7 pm

Answer : To convert 28.7 pm into the basic units:

As $1pm = 10^{-12}m$ .

$\therefore$ $28.7\ pm = 28.7\times10^{-12}m = 2.87\times10^{-11}m$

Question 1.27 Convert the following into basic units:

(ii) 15.15 pm

Answer : To convert 15.15 pm into the basic units:

As $1pm = 10^{-12}m$ .

$\therefore$ $15.15\ pm = 15.15\times10^{-12}m = 1.515\times10^{-11}m$

Question 1.27 Convert the following into basic units:

(iii) 25365 mg

Answer :To convert 25365 mg into basic unit:

As $1mg = 10^{-3}g$ .

$\therefore$ $25365\ mg = 2.5365\times10^4\times10^{-3}g$

Now, as

$1 g= 10^{-3} kg$

$2.5365\times10g = 2.5365\times10\times10^{-3} kg$

$\therefore 25365\ mg = 2.5365\times 10^{-2}kg$

Question 1.28 Which one of the following will have the largest number of atoms?
(i) 1 g Au (s)
(ii) 1 g Na (s)
(iii) 1 g Li (s)
(iv) 1 g of Cl 2 (g)

Answer : Calculating and then comparing for each:

(i) 1 g of Au will contain:

$= \frac{1}{197}\ mol= \frac{1}{197}\times6.022\times10^{23}\ atoms.$

(ii) 1 g of Na will contain:

$= \frac{1}{23}\ mol= \frac{1}{23}\times6.022\times10^{23}\ atoms.$

(iii) 1 g of Li will contain:

$= \frac{1}{7}\ mol= \frac{1}{7}\times6.022\times10^{23}\ atoms.$

(iv) 1 g of Cl 2 will contain:

$= \frac{1}{71}\ mol= \frac{1}{71}\times6.022\times10^{23}\ atoms.$

Clearly, we can compare and say that the number of atoms in 1g of Li has the largest.

Question 1.29 Calculate the molarity of a solution of ethanol in water, in which the mole fraction of ethanol is 0.040 (assume the density of water to be one) .

Answer : Given that the mole fraction of ethanol in water is $0.040$ .

$Mole\ fraction\ of\ ethanol\ = \frac{Number\ of\ moles\ of\ ethanol}{Total\ number\ of\ moles\ of\ solution}$

$\Rightarrow X_{ethanol} = \frac{n_{ethanol}}{n_{water}+n_{ethanol}} = 0.040$

To find the molarity we must have to find the number of moles of ethanol present in 1litre of solution.

Assuming the density of water to be $1 kg\ m^3$ .

Therefore, water is approximately equal to 1Litre.

The number of moles in 1L of water:

$= \frac{1000g}{18g}\ mol^{-1} = 55.55\ moles$

So, substituting in place of $n_{water} = 55.55$ in above equation we get,

$\Rightarrow X_{ethanol} = \frac{n_{ethanol}}{55.55+n_{ethanol}} = 0.040$

$\Rightarrow 0.96\times n_{ethanol} = 55.55\times0.040$

$n_{ethanol} = 2.31\ mol$

So, $2.31\ moles$ are present in 1L of solution.

Hence, the molarity of the solution is $2.31\ M$

Question 1.30 What will be the mass of one 12 C atom in g?

Answer :

As we know the mass of 1 mole of $_{}^{12}C\textrm{}$ atoms or $6.022\times10^{23}$ number of atoms is 12grams . Hence, the mass of one $_{}^{12}C\textrm{}$ atom will be: $\Rightarrow \frac{12}{6.022\times10^{23}g} = 1.9927\times10^{-23}g$

Question 1.31 How many significant figures should be present in the answer of the following calculation?

(i) $\frac{0.02856\times 298.12\times 0.112}{0.5785}$

Answer :To find the number of significant figures that would be present in the answer, we will be finding the least precise term, having the least significant figures.

Here, the least precise term is 0.112 having only 3 significant digits.

Therefore, there will be 3 significant figures in the calculated answer.

Question 1.31 How many significant figures should be present in the answer of the following calculation?

(ii) $5\times 5.364$

Answer : Here, $5.364$ is having 4 significant digits.

Therefore, after multiplying by 5 the answer would also have the same 4 significant figures .

Question 1.31 How many significant figures should be present in the answer of the following calculation?

(iii) 0.0125 + 0.7864 + 0.0215

Answer : Here, the least number of decimal places in each term is four.

Therefore, the calculation would also have the same 4 significant figures .

Question 1.32 Use the data given in the following table to calculate the molar mass of naturally occuring argon isotopes:

Answer : For different isotopes of argon, we have given their naturally occurring abundances.

So, to calculate the molar mass:

Multiply the isotopic molar mass with their abundance to get the molar mass, and then add all of them to get,

$\therefore Molar\ Mass\ of Argon = \sum m_{i}A_{i}$

$= (35.96755\times0.337)+ (37.96272\times0.063)+(39.9624\times99.600)$

$= 39.948g\ mol^{-1}$

Question 1.33 Calculate the number of atoms

(i) 52 moles of Ar

Answer : As 1 mole of air contains $6.022\times10^{23}$ atoms.

Therefore, 52 moles of Ar will contain $52\times6.022\times10^{23}$ atoms.

$\Rightarrow 3.131\times10^{25}\ atoms$ .

Question 1.33 Calculate the number of atoms

(ii) 52 u of He.

Answer :

As 1 atom of He weights 4 u.

Hence the number of atoms of He present in 52 u will be:

$=\frac{52\ u}{4\ u} = 13\ atoms$

Question 1.33 Calculate the number of atoms

(iii) 52 g of He.

Answer : As 1 mole of He weights 4g and contains $6.022\times10^{23}$ atoms.

Therefore, $52g$ will contain $\frac{52}{4}moles = 13\ moles$ .

Hence, the number of atoms will be $13\times6.022\times10^{23}\ atoms$ .

$\Rightarrow 7.8286\times10^{24}\ atoms$ .

Question 1.34 A welding fuel gas contains carbon and hydrogen only. Burning a small sample of it in oxygen gives 3.38 g carbon dioxide, 0.690 g of water and no other products. A volume of 10.0 L (measured at STP) of this welding gas is found to weigh 11.6 g. Calculate

(i) empirical formula

Answer :The amount of carbon in $3.38 g$ of $CO_{2}$ :

$= \frac{12}{44}\times3.38g = 0.9218g$

The amount of hydrogen in $0.690g$ of $H_{2}O$ :

$= \frac{2}{18}\times0.690g = 0.0767g$

The compound contains only C and H,

Therefore, the total mass of the compound will be:

$= 0.9218+0.0767 = 0.9985g$

Now, the percentage of Carbon in the compound:

$= (\frac{0.9218}{0.9985})\times100 = 92.32$

and the percentage of Hydrogen in the compound:

$= (\frac{0.0767}{0.9985})\times100 = 7.68$

Now, the empirical formula,

Moles of carbon in the compound:

$= \frac{92.32}{12} = 7.69$

Moles of hydrogen in the compound:

$= \frac{7.68}{1} = 7.68$

So, the simplest molar ratio will be $= 7.69:7.68 = 1:1$

Therefore, the empirical formula is $CH$ .

Question 1.34 A welding fuel gas contains carbon and hydrogen only. Burning a small sample of it in oxygen gives 3.38 g carbon dioxide, 0.690 g of water and no other products. A volume of 10.0 L (measured at STP) of this welding gas is found to weigh 11.6 g. Calculate

(ii) molar mass of the gas

Answer : Weight of 10 Litres of the gas at S.T.P. is $11.6g$

Weight of 22.4 Litres of gas at S.T.P. will be:

$= \frac{11.6g}{10.0L}\times22.4L$

$= 25.984\ g\ mol^{-1}$

$\approx 26\ g\ mol^{-1}$

Question 1.34 A welding fuel gas contains carbon and hydrogen only. Burning a small sample of it in oxygen gives 3.38 g carbon dioxide, 0.690 g of water and no other products. A volume of 10.0 L (measured at STP) of this welding gas is found to weigh 11.6 g. Calculate

(iii) molecular formula.

Answer : As we know the empirical formula from the previous part is $CH$ .

The mass of empirical formula $CH$ $= 12+1 =13$ .

Therefore,

$n = \frac{Molecular\ Mass}{Empirical\ Formula}$

$\Rightarrow \frac{26}{13} =2$

Therefore, Molecular formula is $C_{2}H_{2}$ .

Question 1.35 Calcium carbonate reacts with aqueous HCl to give CaCl 2 and CO 2 according to the reaction,

$CaCO_3 (s) + 2 HCl (aq) \rightarrow CaCl_2 (aq) + CO_2 (g) + H_2 O(l)$

What mass of CaCO 3 is required to react completely with 25 mL of 0.75 M HCl?

Answer : $0.75M$ $HCl$ contains $0.75\ mole$ in $1000mL$ of solution.

Or, $0.75\times36.5g = 24.375g$ of $HCl$ in $1000mL$ solution.

Therefore,

Mass of $HCl$ in $25mL$ of $0.75M\ HCl$ :

$= \frac{24.375}{1000}\times25 g = 0.6844g$

so, from the given chemical equation,

$CaCO_3 (s) + 2 HCl (aq) \rightarrow CaCl_2 (aq) + CO_2 (g) + H_2 O(l)$

1 mole of $CaCO_{3}(s)$ i.e., $100g$ reacts with 2 moles of $HCl(aq)$ i.e., $73g$ .

Therefore, $0.6844g$ $HCl$ reacts completely with $CaCO_{3}$ to give:

$=\frac{100}{73}\times 0.6844g = 0.938 g$

Question 1.36 Chlorine is prepared in the laboratory by treating manganese dioxide (MnO 2 ) with aqueous hydrochloric acid according to the reaction

$4 HCl (aq) + MnO_2 (s) \rightarrow 2H_2 O (l) + MnCl_2 (aq) + Cl_2 (g)$

How many grams of HCl react with 5.0 g of manganese dioxide?

Answer :Molar mass of $MnO_{2}$ is $55+32 g = 87g$ .

Here from the reaction 1 mole of $MnO_{2}$ reacts with 4 moles of $HCl$ ,

i.e., $4\times36.5g =146g$ of $HCl$ .

Therefore, $5.0 g$ of $MnO_{2}$ will react with $HCl$ :

$= \frac{146}{87}\times5.0g = 8.40g.$

Class 11 Chemistry NCERT Chapter 1: Higher Order Thinking Skills (HOTS) Questions

Question: An unknown organic compound containing ‘amine’ as the functional group produces nitrogen gas on treatment with nitrous acid. On exhaustive methylation with methyl iodide, it forms a quaternary salt that contains 59.07% iodine. Evaluate the molecular mass of the quaternary salt and the number of carbon atoms in the unknown organic compound. Given, the atomic mass of $I=127 \mathrm{~g}$

(If the molecular mass of the quaternary salt is 123 g and the number of carbon atoms in the unknown organic compound is 4, report your answer as 123.04).

Answer:

The evolution of nitrogen upon treatment of the organic compound with nitrous acid suggests that the compound is a primary amine.

Now, 59.07 of iodine is present in 100 g of compound.

$\Rightarrow$ 127 g of iodine will be present in $\frac{100 \times 127}{59.07}=215 \mathrm{~g}$ of compound.

Thus, the molecular mass of quaternary salt =215 g.

Now, the quarternary ammonium salt is $R N\left(\mathrm{CH}_3\right)_3 I$

Mass balance gives,

R+14+45+127 = 215

Thus , R = 29

Therefore, R is C₂H₅

Thus, the unknown organic compound is ethanamine, which contains two carbon atoms.

Hence, the answer is (215).

Approach to Solve Questions of Some Basic Concepts of Chemistry

To solve the NCERT Chapter 1 questions effectively, students have to focus on understanding the fundamental concepts, along with practicing questions given in examples and exercises. Students should utilise the sources and content provided online to ensure a complete understanding of the subject. Here are a few things that students should keep in mind while preparing:

• Read the chapter thoroughly and familiarize yourself with the definitions and theorems
• Work through examples and apply the same approach to solve exercises
• Test yourself with different sets of questions
• Identify your weaknesses and work on them

NCERT Topics Some Basic Concepts of Chemistry

1.1 Importance of Chemistry

1.2 Nature of Matter

1.3 Properties of Matter and their Measurement

1.4 Uncertainty in Measurement

1.5 Laws of Chemical Combinations

1.6 Dalton’s Atomic Theory

1.7 Atomic and Molecular Masses

1.8 Mole Concept and Molar Masses

1.9 Percentage Composition

1.10 Stoichiometry and Stoichiometric Calculations

What Extra Should Students Study Beyond NCERT for JEE/NEET?

Class 11 Chapter 1 formulas

1. The mass percent of an element
$\\Mass\:percent\:of\: an\: element =\frac{Mass \:of\:that\:element\:in \:the\:compound\times 100}{molar \:mass\:of\:the\:compound}$
2. Mass percent
$\\Mass\:percent =\frac{Mass \:of\:solute\times 100}{mass\:of\:solution}\\$
3. Mole fraction
$\\Mole\: fraction \:of \:A =\frac{No. \:of\:moles\:of\:A}{No.\:of\:moles\:of\:solutions}=\frac{n_A}{n_A+n_B}$
4. Molarity(M)
$\\M =\frac{No. \:of\:moles\:of\:solute}{Volume\:of\:solution\:in\:litres}$
5. Molality(m)
$\\m =\frac{No. \:of\:moles\:of\:solute}{Mass\:of\:solvent\:in\:Kg}$

Why NCERT Chapter 5 is important for JEE/NEET preparation?

The NCERT Chapter 1, Some Basic concepts of chemistry is important for JEE/NEET preparation because of the following reasons:

• Chapter includes fundamental concepts required to solve complex problems
• It deals with atoms, molecules, elements and compounds
• It introduces basic laws of chemistry, viz., law of conservation of mass, and the law of definite proportions etc.
• Mole concept helps understand stochiometry problems and calculating molar masses

NCERT Solutions For Class 11 Chemistry

NCERT Solutions for Class 11 Subject Wise

Also, Check NCERT Books and NCERT Syllabus here