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Ph of Acids and Bases - Definition, Examples, Limitations, FAQs

Ph of Acids and Bases - Definition, Examples, Limitations, FAQs

Edited By Team Careers360 | Updated on Jul 02, 2025 04:38 PM IST

The theory of pH was introduced by Danish chemist Soren Peder Lauritz Sorensen in the year of 1909. Sorensen developed the pH scale to measure the acidity or Basicity of a solution more accurately. The scale that he discovered to measure the acidity and basicity of the compound expresses the concentration of hydrogen ions in the solution, the lower pH is the indicator of higher Hydrogen ion concentration and then that of higher pH indicates the lower hydrogen ion concentration, and a very basic namely Alkaline Solution.

This Story also Contains
  1. Ph Of Acids And Bases
  2. Some Solved Examples
  3. Summary
Ph of Acids and Bases - Definition, Examples, Limitations, FAQs
Ph of Acids and Bases - Definition, Examples, Limitations, FAQs

Ph Of Acids And Bases

what is pH

pH is also referred to as the potential or power of hydrogen. Mathematically, it can be represented as follows:

pH=log10[H3O+]

If the solution is neutral, then:
Kw = [H3O+][OH-]
From the ionic product of water, we know:
Kw = 10-14
[H3O+] = [OH-] = x (since solution is neutral)
Thus, 10-14 = Kw = x2
x = 10-7
Now, [H3O+] = 10-7
Thus, pH = - log10(H3O+) = - log10(10-7) = 7

For Acidic solutions: For Basic solutions:

For acidic solutions, we must have [H3O+] > [OH-] For basic solutions, we must have [H3O+] < [OH-]
Thus, [H3O+] > 10-7 Thus, [H3O+] < 10-7
Thus, [H3O+] for acids can be 10-6, 10-5, 10-4, etc. Thus, [H3O+] for basics can be 10-8, 10-9, 10-10, etc.
Thus, pH of acids can be 6, 5, 4, etc. Thus, pH of basics can be 8, 9, 10, 11, etc.
Hence, pH of acidic solutions is less than 7 Hence, pH of basic solutions is greater than 7

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Background wave

pH depends upon the temperature

We know from the ionic product of water that at 630C, the value of Kw = 10-13.
For a neutral solution, we know the:
[H3O+]=[OH]Kw=x2x=1013=106.5[H3O+]=106.5pH=log10(106.5)=6.5

Hence, pH depends upon the temperature

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pH of Strong Acids

Strong acids are those acids that dissociate completely in solutions. For example:

  • 2 x 10-3 M HNO3
    Since HNO3 is a strong acid, thus it will dissociate completely into H+ and OH- ions as follows:
    HNO3H++NO3Thus, [H+]=2×103M (given) pH=log10(2×103)pH=log10(2)log10(103)pH=0.30+3=2.7
    Thus, the pH of HNO3 is 2.7
  • 10-4 M H2SO4
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Since H2SO4 is a strong acid, thus it will dissociate completely into H+ and OH- ions as follows:

H2SO42H++SO42 Thus, [H+]=2×104MpH=log10(2×104)pH=log10(2)log10(104)pH=0.30+4=3.7
Thus, the pH of H2SO4 is 3.7

NOTE: If molarity(N) of solution is not given but normality(N) is given, then molarity can be calculated using the following formula:

N = M x n
where n is the number of moles

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the pH of Weak Acids

Weak acids are those acids that dissociate partially in solutions. For example:

  • 8 M HA (Ka =2 x 10-8)
    The chemical equation for the dissociation of weak acid HA is as follows:
    HAH++A
    Initial: 8M 0 0
    Equil: 8 - 8? 8? 8?
    The equilibrium constant Ka for the weak acid is given as follows:
    Ka=[H+][A][HA]=8α.8α8(1α)=8α21αKa=8α2( as (1α)1) Thus, α=Ka8=2×1088=1084=0.5×104[H+]=8×0.5×104=4×104pH=log104+4pH=0.60+4=3.4
    Thus, the pH of this given acid = 3.4
  • 0.002N CH3COOH(? = 0.02)
    The chemical equation for the dissociation of CH3COOH is as follows:
    CH3COOHCH3COO+H+
    Initial: c 0 0
    Equil: c - c? c? c?
    The equilibrium constant Ka for the weak acid is given as follows:
    Ka=[CH3COOH][H+][CH3COOH]=cαcαc(1α)=cα21αKa=cα2( as (1α)1)
    Now, as we have given :
    c=0.002 N or 0.002M( Normality = Molarity, as n factor =1)α=2100=0.02
    Thus, [H+]=0.002×0.02=4×105
    pH=log10(4×105)pH=5log4=4.4
    Thus, the pH of acetic acid = 4.4

Recommended topic video on (pH Of Acids And Bases)


Some Solved Examples

Example.1

1. Which one of the following statements is not true?

1)The conjugate base of H2PO4is HPO42
2) pH+pOH=14 for all aqueous solutions.
3) (correct) The pH of 1×108MHCL is 8
4)96,500 coulombs of electricity when passed through a CuSO4 solution deposits 1 gram equivalent of copper at the cathode.

Solution

Value of pH -An acidic solution has pH<7, The basic solution has pH>7, Neutral solution has pH=7.pHacid cannot exceed 7 Here we should also consider [H+]that comes from H2O Now [H+]=[H+]from HCl+[H+]from H2O
[H]+=108+107[H]+=108+10×108[H]+=11×108
pH=log(11×108)=6.9587

Hence, the answer is the option (3).

Example.2

2. An alkali is titrated against an acid with methyl orange as an indicator, which of the following is a correct combination?

1)Base Acid End Point

Strong Strong Pink to colorless

2)Base Acid End Point

Weak Strong Colourless to pink

3)Base Acid End Point

Strong Strong Pinkish red to yellow

4) (correct)Base Acid End Point

Weak Strong Yellow to pinkish red

Solution

As we learned in

Value of p(H) -

Acidic solution has p(H) < 7

Basic solution has p(H) > 7

Neutral solution has p(H) = 7

Methyl orange changes its color in acidic medium when pH is around 3 to 5 and its colour is yellow in basic medium and red in acidic medium so it is used to titrate weak base with strong acid.

Hence, the answer is an option (4).

Example.3

3. Among the following acids which have the lowest pKa value?

1) CH3COOH
2) (CH3)2CHCOOH
3) (correct) HCOOH
4) CH3CH2COOH

Solution

The higher the pKa value, the weaker is the acid. Hence, stronger acid has lower pKa value.

In the given options, HCOOH is the strongest acid.

Hence, the answer is the option (3).

Example.4

4.Hydrogen ion concentration in mol/L in a solution of pH=5.4 will be

1)3.98×108

2)3.88×106

3)3.68×106

4) (correct)3.98×106

Solution

The pH scale -

Hydronium ion concentration in molarity is more conveniently expressed on a logarithmic scale known as the pH scale.

The pH of a solution is defined as the negative logarithm to base 10 of the activity of hydrogen ion

pH=log[H+]

[H+]=antilog(pH)=antilog(5.4)=3.98×106

Hence, the answer is the option (4).

Example.5

5.How many litres of water must be added to 1 litre of aqueous solution of HCl with a pH of 1 to create an aqueous solution with pH of 2 ?

1) (correct)9.0 L

2)0.1 L

3)0.9 L

4)2.0 L

Solution

If molarity(N) of solution is not given but normality(N) is given, then molarity can be calculated using the following formula:

N = M x n
where n is the number of moles

Initial PH=2=1log[H+]2

[H+]1×1=[H+]2×V

101×1=102×V

V=10L

Added water =10-1=9L

Hence, the answer is the option (1).

Summary

PH is describe as the negative logarithm (base 10) of the hydrogen ion concentration. Acids are substances that donate hydrogen ions (protons) to any solution. They have a pH of less than 7. Strong Acids are those that are completely dissociated in water, such as hydrochloric acid (HCl) and sulfuric acid (H2SO4). They have a low pH, which is close to 0. Weak acids such as acetic acid (CH3COOH) partially dissociate in water. They have a pH closer to 7 but still less than 7 . Examples: Lemon juice, vinegar, and battery acid. Bases are substances that accept hydrogen ions or donate hydroxide ions (OH-) in solution. They have a pH greater than 7. Strong bases such as sodium hydroxide (NaOH) and potassium hydroxide (KOH) are completely dissociated in water. They have a high pH, often close to 14. Weak Bases are partially dissociated in water, such as ammonia (NH3). They have a pH almost equal to 7 but still more than 7.

NCERT Chemistry Notes:

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Frequently Asked Questions (FAQs)

1. 1. How does the pH scale in chemistry work?

pH determines the number of hydrogen ions in a solution, as well as its acidity or alkalinity. Normally, a pH scale ranges from zero to fourteen an alkaline solution is one with a pH greater than 7 and an acidic one with a pH of less than 7 at 25 °C.

2. 2. What is the optimal pH for the human body?

pH 7.2 is the optimum level for our blood and body tissues. Acidosis is a condition of increased acidity in the blood and body tissues.

3. 3.The full form of pH is what?

Hydrogen potentials are known as PH. A hydrogen ion concentration indicates a solution's hydrogen ion concentration. A measure of how acidic or alkaline a solution is. There are 14 pH values on a pH scale.

4. 4. What role does pH play in the water?

An acidic or basic water content determines the pH. An acidic solution contains more hydrogen ions, and a basic solution contains more hydroxyl ions. pH is an important indicator of the chemical state of water since it can be changed by chemicals.

5. 5.What happens if you have an excessively high pH?

pH increases with an increase in alkalinity. Acidosis is a condition where the blood acid levels become too high. If the blood is too alkaline, this is called alkalosis. Acidosis and alkalosis of the respiratory system can cause the lungs to malfunction.

pH = -log aH+

6. What is pH and why is it important in chemistry?
pH is a measure of the acidity or basicity of a solution on a scale of 0 to 14. It's important because it helps us understand the chemical properties of substances, predict reactions, and is crucial in many biological and industrial processes. pH affects everything from the taste of food to the health of ecosystems.
7. How does the pH scale work?
The pH scale ranges from 0 to 14. A pH of 7 is neutral, below 7 is acidic, and above 7 is basic (alkaline). Each unit change represents a tenfold change in hydrogen ion concentration. For example, a solution with pH 5 is ten times more acidic than a solution with pH 6.
8. Why does pure water have a pH of 7?
Pure water has a pH of 7 because it contains equal concentrations of hydrogen ions (H+) and hydroxide ions (OH-). This balance results in a neutral solution. However, even slight contamination can shift the pH of water.
9. Can pH values be negative or greater than 14?
Yes, although rare, pH values can be negative or greater than 14 in extremely concentrated acidic or basic solutions. These extreme values are theoretical extensions of the standard pH scale and are not commonly encountered in everyday situations.
10. What's the difference between a strong acid and a weak acid?
Strong acids completely dissociate in water, releasing all their hydrogen ions. Weak acids only partially dissociate. As a result, strong acids have lower pH values and are more reactive than weak acids of the same concentration.
11. How does temperature affect pH?
Temperature can affect pH measurements. Generally, as temperature increases, pH decreases slightly for neutral and acidic solutions. This is because higher temperatures increase the dissociation of water molecules, producing more hydrogen ions.
12. What is a buffer solution and how does it work?
A buffer solution resists changes in pH when small amounts of acid or base are added. It typically consists of a weak acid and its conjugate base (or vice versa). The buffer components can absorb extra H+ or OH- ions, maintaining a relatively stable pH.
13. Why doesn't the pH scale start at 1 instead of 0?
The pH scale starts at 0 because it's based on the negative logarithm of hydrogen ion concentration. A 1 molar solution of a strong acid has a pH of 0. Starting at 1 would exclude some very strong acid solutions that can exist.
14. How do indicators work to measure pH?
pH indicators are weak acids or bases that change color at specific pH values. They work by changing their molecular structure in response to H+ concentration, which results in a visible color change. Different indicators work best for different pH ranges.
15. What's the relationship between pH and pOH?
pH and pOH are complementary measures. In aqueous solutions at 25°C, pH + pOH = 14. This relationship comes from the ion product of water (Kw). As pH increases, pOH decreases, and vice versa.
16. How does CO2 affect the pH of water?
When CO2 dissolves in water, it forms carbonic acid (H2CO3), which dissociates to release H+ ions. This lowers the pH of the water, making it more acidic. This process is important in understanding ocean acidification and the carbon cycle.
17. Why do strong acids and bases conduct electricity better than weak ones?
Strong acids and bases conduct electricity better because they dissociate completely in water, producing more ions. More ions mean more charge carriers, resulting in better electrical conductivity. Weak acids and bases only partially dissociate, producing fewer ions.
18. How does pH affect enzyme activity?
Enzymes have an optimal pH range where they function best. Outside this range, the enzyme's structure can change (denature), reducing its effectiveness. Different enzymes have different optimal pH ranges, often corresponding to their natural environment in the body.
19. What's the difference between an acid-base titration and a pH titration?
An acid-base titration determines the concentration of an unknown acid or base by reacting it with a known concentration of base or acid. A pH titration specifically measures the pH changes during this process, often using a pH meter or indicator. pH titration provides more detailed information about the reaction progress.
20. How does the concept of pH apply to non-aqueous solutions?
While pH is traditionally defined for aqueous solutions, the concept can be extended to non-aqueous solvents. However, the scale and interpretation may differ. In non-aqueous solutions, we often use the concept of acidity functions, which are analogous to pH but specific to the solvent system.
21. What is the Henderson-Hasselbalch equation and how is it used?
The Henderson-Hasselbalch equation relates the pH of a buffer solution to the pKa of the weak acid and the concentrations of the acid and its conjugate base. It's used to calculate the pH of buffer solutions or to determine the ratio of acid to base needed to create a buffer with a specific pH.
22. How does pH affect the solubility of gases in water?
Generally, the solubility of gases decreases as pH increases. This is because many gases form weak acids when dissolved in water. As the solution becomes more basic (higher pH), these acids dissociate more, reducing the solubility of the gas according to Le Chatelier's principle.
23. What is a zwitterion and how does it relate to pH?
A zwitterion is a molecule with both positive and negative electrical charges. Many amino acids exist as zwitterions at certain pH values. The pH at which a molecule exists as a zwitterion is called its isoelectric point, where it has no net electrical charge.
24. How does pH affect the toxicity of pollutants in water?
pH can significantly influence the toxicity of pollutants in water. Many pollutants become more toxic at certain pH levels. For example, ammonia becomes more toxic at higher pH levels, while some heavy metals become more soluble and thus more bioavailable at lower pH levels.
25. What is the role of pH in blood?
Blood pH is tightly regulated, typically between 7.35 and 7.45. This narrow range is crucial for proper functioning of enzymes, protein structure, and many biochemical processes. The body has several buffer systems to maintain this pH, including the bicarbonate buffer system.
26. How does pH affect the taste of food?
pH significantly influences taste perception. Acidic foods (low pH) taste sour, while alkaline foods can taste bitter. pH also affects the intensity of other flavors and can change the color and texture of food. This is why pH is a crucial factor in food science and cooking.
27. What is a pH meter and how does it work?
A pH meter is an electronic device that measures the pH of a solution. It consists of a special glass electrode that's sensitive to H+ ion concentration. The electrode generates a small voltage that varies with pH. The meter converts this voltage into a pH reading, providing a more precise measurement than pH paper or indicators.
28. How does acid rain form and what are its effects?
Acid rain forms when sulfur dioxide and nitrogen oxides (often from industrial emissions) react with water, oxygen, and other chemicals in the atmosphere. This lowers the pH of rainwater, sometimes to as low as 4. Acid rain can harm aquatic ecosystems, damage forests, and corrode buildings and infrastructure.
29. What is the pH of the human stomach and why is it so acidic?
The pH of the human stomach is typically between 1.5 and 3.5, making it highly acidic. This acidity is crucial for breaking down food, activating digestive enzymes, and killing harmful bacteria. The stomach produces hydrochloric acid (HCl) to maintain this low pH.
30. How does pH affect the absorption of drugs in the body?
pH significantly influences drug absorption. Many drugs are weak acids or bases, and their ionization state depends on pH. Non-ionized forms of drugs are generally more lipid-soluble and can pass through cell membranes more easily. The varying pH levels in different parts of the body (e.g., stomach vs. intestines) affect where and how efficiently drugs are absorbed.
31. What is a titration curve and what information can it provide?
A titration curve is a graph showing the change in pH during a titration. It plots pH against the volume of titrant added. From this curve, you can determine the equivalence point (where the moles of acid equal the moles of base), the pKa of the acid or base, and the buffer regions. It also visually represents the pH changes throughout the titration process.
32. How does the pH of soil affect plant growth?
Soil pH affects nutrient availability to plants. Most plants prefer slightly acidic soil (pH 6.0 to 6.8) because this range optimizes the solubility and uptake of essential nutrients. Extremely acidic or alkaline soils can make certain nutrients less available or even toxic to plants, affecting their growth and health.
33. What is the relationship between Ka (acid dissociation constant) and pH?
Ka is related to pH through the equation: pH = -log[H+] = -log(Ka × [HA]/[A-]), where HA is the acid and A- is its conjugate base. A larger Ka indicates a stronger acid, which will result in a lower pH for a given concentration. The pKa (-log Ka) is often used and is equal to the pH at which half of the acid is dissociated.
34. How does pH affect the structure and function of proteins?
pH can significantly alter protein structure and function. Proteins have an optimal pH range where they maintain their native structure. Extreme pH can denature proteins by disrupting hydrogen bonds and altering the ionization of amino acid side chains. This can lead to loss of function, as in enzyme inactivation or protein precipitation.
35. What is the concept of pH in non-aqueous solvents?
While pH is traditionally defined for aqueous solutions, the concept can be extended to non-aqueous solvents. In these cases, we often use the term "acidity function" instead of pH. The scale and interpretation may differ depending on the solvent's properties, such as its ability to donate or accept protons.
36. How does the pH inside cells compare to the pH outside cells, and why is this important?
The pH inside most cells (intracellular pH) is typically around 7.0-7.2, while the pH outside cells (extracellular pH) is usually around 7.4. This pH gradient across the cell membrane is crucial for many cellular processes, including energy production, nutrient transport, and maintaining cell volume.
37. What is the role of pH in the carbon cycle and ocean acidification?
pH plays a critical role in the carbon cycle, particularly in oceans. As atmospheric CO2 levels increase, more CO2 dissolves in the oceans, forming carbonic acid and lowering the pH. This process, known as ocean acidification, can have severe impacts on marine ecosystems, especially organisms with calcium carbonate shells or skeletons.
38. How does pH affect the solubility of ionic compounds?
The solubility of many ionic compounds, especially those containing weak acids or bases, is pH-dependent. For example, the solubility of metal hydroxides typically decreases as pH increases. This principle is used in various applications, from water treatment to selective precipitation in chemical analysis.
39. What is a pH-dependent conformational change in proteins?
A pH-dependent conformational change occurs when alterations in pH cause a protein to change its three-dimensional structure. This can happen due to the protonation or deprotonation of amino acid side chains, which can form or break salt bridges and hydrogen bonds. These changes can activate or inactivate proteins, playing crucial roles in biological processes.
40. How does pH affect the rate of chemical reactions?
pH can significantly influence reaction rates, especially for reactions involving acids or bases. It can affect the concentration of reactive species, alter the activation energy of reactions, and change the reaction mechanism. In some cases, pH changes can catalyze or inhibit reactions.
41. What is the difference between pH and pKa?
pH is a measure of the acidity or basicity of a solution, while pKa is a measure of the strength of an acid. Specifically, pKa is the negative logarithm of the acid dissociation constant (Ka). At a pH equal to the pKa of an acid, half of the acid molecules are dissociated. This relationship is crucial in understanding buffer solutions and acid-base equilibria.
42. How does pH affect the color of natural indicators like anthocyanins in flowers?
Anthocyanins, pigments found in many flowers and fruits, change color depending on pH. In acidic conditions, they appear red; in neutral conditions, purple; and in basic conditions, blue or green. This pH-dependent color change is due to structural changes in the anthocyanin molecules and is the basis for some natural pH indicators.
43. What is the role of pH in wastewater treatment?
pH control is crucial in wastewater treatment. It affects the solubility of contaminants, the efficiency of biological treatment processes, and the effectiveness of chemical treatments. For example, adjusting pH can help precipitate heavy metals or optimize conditions for beneficial bacteria in biological treatment stages.
44. How does pH affect the formation of kidney stones?
The formation of kidney stones is influenced by urine pH. Certain types of stones, like calcium oxalate stones, tend to form in acidic urine, while others, like calcium phosphate stones, form in alkaline urine. Understanding this relationship helps in the prevention and treatment of kidney stones through dietary and medication interventions that alter urine pH.
45. What is the concept of microenvironments in relation to pH?
Microenvironments are small, localized areas within a larger system that can have different pH values from their surroundings. In biological systems, microenvironments can exist within cells, on enzyme surfaces, or in the vicinity of membranes. These localized pH differences can be crucial for specific biochemical reactions or cellular processes.
46. How does pH affect the stability and shelf life of pharmaceuticals?
pH significantly influences the stability of many drugs. It can affect the chemical structure of pharmaceuticals, potentially leading to degradation or loss of efficacy. Many drugs have an optimal pH range for stability, which is why buffers are often used in drug formulations to maintain a consistent pH and extend shelf life.
47. What is the relationship between pH and oxidation-reduction (redox) reactions?
pH can significantly influence redox reactions. In many redox reactions, H+ ions are either consumed or produced, directly linking them to pH changes. Additionally, the standard reduction potentials of many half-reactions are pH-dependent. This relationship is crucial in understanding processes like corrosion and designing electrochemical cells.
48. How does pH affect the binding of oxygen to hemoglobin?
The binding of oxygen to hemoglobin is pH-sensitive, a phenomenon known as the Bohr effect. As pH decreases (becomes more acidic), hemoglobin's affinity for oxygen decreases, facilitating oxygen release in tissues. This is physiologically important as active tissues produce more CO2 and lactic acid, lowering pH and promoting oxygen release where it's needed most.
49. What is the role of pH in food preservation?
pH plays a crucial role in food preservation by inhibiting microbial growth. Many harmful bacteria cannot survive in highly acidic environments. Lowering the pH of foods through fermentation or the addition of acids is a common preservation technique. Additionally, pH affects the effectiveness of many preservatives and the rate of chemical reactions that can lead to food spoilage.
50. How does pH affect the toxicity and bioavailability of heavy metals in the environment?
pH significantly influences the toxicity and bioavailability of heavy metals in soil and water. Generally, as pH decreases, heavy metals become more soluble and thus more bioavailable to organisms. This increased bioavailability often leads to increased toxicity. Understanding this relationship is crucial in environmental remediation and risk assessment of contaminated sites.
51. What is the concept of proton pumps and how do they relate to pH regulation in cells?
Proton pumps are proteins in cell membranes that actively transport hydrogen ions (protons) across the membrane, often against their concentration gradient. They play a crucial role in maintaining pH gradients across cellular compartments and in processes like ATP synthesis. In the stomach, proton pumps in parietal cells are responsible for creating the highly acidic environment necessary for digestion.
52. How does pH affect the efficacy of disinfectants and antiseptics?
The efficacy of many disinfectants and antiseptics is pH-dependent. Some work best in acidic conditions, while others are more effective in alkaline environments. pH can affect the chemical structure of the disinfectant, its ability to penetrate microbial cells, and the overall antimicrobial activity. Understanding this relationship is crucial in healthcare and sanitation practices.
53. What is the role of pH in the brewing process of beer and wine?
pH plays a critical role in brewing and winemaking. It affects enzyme activity during mashing in beer production, influences yeast performance during fermentation

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