Difference Between Molar Mass and Molecular Mass with FAQs

Edited By Team Careers360 | Updated on Jul 02, 2025 04:55 PM IST

Molar mass

Molar mass is also known by the name relative molar mass which can be defined as the smallest mass unit of any compound with respect to the one twelfth of the mass of a carbon atom.

What is the molar mass unit?

Unit of molar mass can be defined by two units one is standard unit and other one is S.I. unit. Standard unit of molar mass is gmol^-1 and S.I. unit is given by Kgmol^-1, S.I. unit of molar mass is rarely seen.

Molar mass can also be defined on the basis of mole concept where mole of any substance is defined as a substance or particle can be defined as containing exactly NH₃=14+3×1=17u particles which may be atoms, molecules or ions where 6.02214076×10²³ is known as Avogadro's number. Mole can be represented by its symbol called mol. It is generally described as the unit of measurement for amount of substance in SI where SI stands for International System of units. Mole can be easily defined on the basis of Avogadro’s number.

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Molar mass
Difference between molar mass and atomic mass
Molecular mass
Difference between molar mass and molecular mass:

The number of atoms in 12g (0.012 kg) of isotope of carbon i.e. ¹²C equals the number of particles in 1 mole of the material. One of the most crucial facts to remember is that any substance mole always contains the same amount of entities, regardless of the substance.

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Molar mass formula

Calculating the atomic, molecular, and formula masses of atoms, molecules, and other compounds is fine, but since we can't weigh a single particle, these masses are only of limited utility. We must express chemical quantities at the macroscopic level to make mass measurements useful. Molar mass, or the mass in grams of one mole of a substance, serves as a link between the particle and macroscopic levels. The units of molar mass are grams per mole, as defined by its definition. The defining equation of molar mass in mathematics is

Molar mass = mass/mole = g/mol

Carbon-12 is directly or indirectly related to the definitions of atomic mass, mole, and molar mass. This leads to two key points which are described as:

The atomic mass of one carbon-12 atom is exactly 12 atomic mass units.
One mole of carbon-12 atoms has a mass of exactly 12 grams, and its molar mass is also exactly 12 grams per mole.

Difference between molar mass and atomic mass

Carbon-12's atomic mass and molar mass are quantitatively equivalent. Only the units are different: atomic mass is measured in atomic mass units, whereas molar mass is measured in grams per mole. The atomic and molar masses of elements, molecular masses and molar masses of molecular substances, and formula masses and molar masses of ionic compounds all have the same relationship.

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Molecular mass

The molecular mass of a substance is defined as the sum of the mass of every single molecule present in it. We can easily calculate it by just adding the atomic mass units of the individual atoms in order to calculate the molecular mass of a molecule. Dalton is the most common unit of molecular mass, and it is symbolized by Da and one Dalton is equal to one unit i.e. 1Da=1u.

Molecular mass formula

The molecular mass can be easily calculated by multiplying the number of atoms in each molecule to the given molar mass of atoms. The quantity of mass associated with a molecule is known as its molecular mass. The mass of each atom multiplied by the number of atoms of the element present in the molecule can be computed. By taking the example of water we can define a molecular mass formula: water generally contains two atoms of hydrogen and one atom of oxygen. The average atomic mass of hydrogen multiplied by two plus the atomic mass of oxygen equals the mass of a water molecule. The molecular mass of elements is determined by the atoms that make up the molecule.

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Difference between molar mass and molecular mass:

Molar mass and molecular mass can be differentiated on the basis of following factors:

1. Molar mass is generally referred to with the term mole whereas molecular mass is associated with mass of molecules present in a compound.

2. Molar mass is also known by the name molecular weight whereas molecular mass is known to us for determining the mass of a single molecule.

3. Molar mass is expressed in units like in g/mol for higher calculations while molecular mass is expressed in amu i.e. atomic mass unit.

4. Molar mass is generally defined as the mass of Avogadro number of atoms, molecules or compounds while molecular mass is defined as the sum of atomic masses of all the atoms which are present in the given molecule or compound.

5. In case of molar mass the measurement is given to compounds, atoms or molecules while in case of molecular mass we only determine molecules.

6. Molar mass is generally less accurate as compared to molecular mass while molecular mass is accurate and helps in higher calculations.

Is molar mass and molecular mass are same?/ Is molar mass and molecular mass same and the main examples of molar mass and molecular mass can be shown as follows:

Molar mass: The example of molar mass can be considered by taking the example of mass of 1 mole of oxygen which is 15.9994 grams which is exactly equal to molar mass which is given by 15.9994 g/mol.

Molecular mass: Molecular mass of water is just add up the hydrogen and oxygen atoms by summing them up i.e. in case of H₂O hydrogen atoms are available and one oxygen atom is present and we know that the mass of a hydrogen atom is 1 u and mass of one oxygen atom is 16 u. Now molecular mass of water is calculated by:

1640229605447

Hence on the basis of these points we can easily differentiate between molar mass and molecular mass.

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Frequently Asked Questions (FAQs)

1. What is Avogadro number?

Avogadro's number or Avogadro constant is generally gives us the relation between the number of constituent molecules, atoms or ions in a given sample with their corresponding volume given in that sample.

2. Is atomic mass important in chemistry?

In chemistry atomic mass is very important as it provides us the connection between moles and masses i.e. number of atoms in any substance that’s why it get greater importance as we come to know about how many atoms are present in any compound through its atomic mass.

3. Define mole.

Mole can be represented by its symbol called mol. It is generally described as the unit of measurement for the amount of substance in SI where SI stands for International System of units. Mole can be easily defined on the basis of Avogadro’s number.

4. Difference between molecular mass and equivalent mass.

Equivalent mass is defined as the molar mass of substance which is divided by n number of equivalents present in the substance.

5. Why is it important to distinguish between molar mass and atomic mass?

It's important to distinguish between molar mass and atomic mass because they refer to different quantities. Atomic mass is the mass of a single atom, while molar mass is the mass of one mole of atoms or molecules. Understanding this difference is crucial for accurate calculations in chemistry.

6. How does the concept of molar mass apply to solutions?

In solutions, molar mass is used to calculate molarity (moles of solute per liter of solution) and other concentration units. It's essential for preparing solutions of specific concentrations and for stoichiometric calculations involving solutions.

7. Can molar mass be used to identify an unknown compound?

Yes, molar mass can be a useful tool in identifying unknown compounds. By determining the molar mass experimentally and comparing it to known values, or by using it in conjunction with elemental analysis, one can often identify or at least narrow down the possibilities for an unknown compound.

8. Why is molar mass important in colligative properties?

Molar mass is crucial in understanding colligative properties because these properties depend on the number of particles in solution, not their identity. Knowing the molar mass allows us to calculate the number of moles of solute, which directly affects properties like boiling point elevation and freezing point depression.

9. How does molar mass affect osmotic pressure?

Molar mass is inversely related to osmotic pressure. For a given mass concentration, solutions of substances with lower molar masses will have more particles and thus higher osmotic pressures compared to solutions of substances with higher molar masses.

10. What is the main difference between molar mass and molecular mass?

Molar mass is the mass of one mole of a substance, while molecular mass is the mass of a single molecule. Molar mass is expressed in grams per mole (g/mol), whereas molecular mass is typically given in atomic mass units (amu) or daltons (Da).

11. Can molar mass and molecular mass ever be numerically equal?

Yes, molar mass and molecular mass can be numerically equal, but they have different units. For example, the molecular mass of H2O is about 18 amu, and its molar mass is about 18 g/mol. The numerical values are the same, but the units differ.

12. How does the presence of isotopes affect molar mass and molecular mass?

Isotopes affect both molar mass and molecular mass. For molar mass, the average atomic mass of each element (considering all naturally occurring isotopes) is used. For molecular mass, the mass of specific isotopes in a particular molecule is considered.

13. Can a substance have a molecular mass but not a molar mass?

No, all substances have both a molecular mass and a molar mass. Even for elements that exist as single atoms (like noble gases), we can still calculate both values. The molecular mass would be the mass of a single atom, while the molar mass would be the mass of one mole of those atoms.

14. Why is molecular mass sometimes called formula mass?

The term "formula mass" is used when referring to ionic compounds that don't form discrete molecules. It represents the mass of one formula unit of the compound, analogous to molecular mass for molecular compounds.

15. Why is molar mass important in stoichiometric calculations?

Molar mass is crucial in stoichiometry because it allows us to convert between the number of moles and the mass of a substance. This conversion is essential for balancing chemical equations and determining the quantities of reactants and products in chemical reactions.

16. How does the concept of molar mass relate to Avogadro's number?

Molar mass is directly related to Avogadro's number. One mole of any substance contains Avogadro's number (6.022 x 10^23) of particles. The molar mass gives us the mass of this specific number of particles for a given substance.

17. How do you calculate molar mass for a compound?

To calculate the molar mass of a compound, sum the molar masses of all atoms in the molecule. For example, for H2O, add the molar mass of two hydrogen atoms and one oxygen atom: (2 x 1.008 g/mol) + (1 x 15.999 g/mol) = 18.015 g/mol.

18. How does the periodic table help in determining molar mass?

The periodic table provides the average atomic mass for each element, which is numerically equal to its molar mass in g/mol. By using these values and the chemical formula, you can easily calculate the molar mass of any compound.

19. Can molar mass be measured experimentally?

Yes, molar mass can be determined experimentally using various methods such as mass spectrometry, vapor density measurements, or colligative property experiments like freezing point depression or boiling point elevation.

20. Can a molecule have a fractional molecular mass?

While individual molecules always have whole number masses (in amu), the average molecular mass can be a fraction due to the presence of isotopes. For example, the average molecular mass of chlorine (Cl2) is about 70.90 amu, not a whole number.

21. How does the concept of molar mass apply to gases?

For gases, molar mass is particularly important in calculations involving the ideal gas law and gas stoichiometry. It allows us to relate the volume of a gas to its mass and number of moles under specific conditions of temperature and pressure.

22. What's the relationship between molar mass and density?

Molar mass and density are related but distinct. Density is mass per unit volume, while molar mass is mass per mole. For a pure substance, you can calculate its density if you know its molar mass and molar volume (volume occupied by one mole of the substance).

23. How does molar mass affect reaction rates in chemistry?

Molar mass can influence reaction rates, particularly in solutions. Generally, molecules with lower molar masses tend to move faster and collide more frequently, potentially leading to faster reaction rates, all other factors being equal.

24. How does molar mass relate to the concept of limiting reagent?

Molar mass is crucial in identifying the limiting reagent in a reaction. By converting the given masses of reactants to moles using their respective molar masses, we can determine which reactant will be completely consumed first, thus limiting the reaction.

25. Can molar mass be used to predict solubility?

While molar mass alone doesn't determine solubility, it can provide some insights. Generally, for similar compounds, those with lower molar masses tend to be more soluble in water due to a more favorable entropy change. However, other factors like polarity and intermolecular forces play crucial roles in solubility.

26. Can molar mass be used to predict the critical point of a substance?

While molar mass alone doesn't determine the critical point, there is often a correlation. Generally, substances with higher molar masses tend to have higher critical temperatures and pressures. However, molecular structure and intermolecular forces also play significant roles.

27. How does molar mass relate to the concept of partial pressure in gas mixtures?

In gas mixtures, molar mass is used to convert between partial pressures and mole fractions. The partial pressure of each gas is proportional to its mole fraction in the mixture, and molar mass is needed to calculate mole fractions from given masses of gases.

28. Can molar mass be used to predict the conductivity of a solution?

While molar mass alone doesn't determine conductivity, it can provide some insights. For electrolyte solutions of the same concentration, those with lower molar mass solutes often have higher conductivities because they produce more ions per unit mass. However, the degree of dissociation and ion mobility are more direct factors affecting conductivity.

29. How does molar mass relate to the concept of empirical and molecular formulas?

Molar mass helps distinguish between empirical and molecular formulas. The empirical formula represents the simplest whole-number ratio of atoms in a compound, while the molecular formula shows the actual number of atoms. The ratio of the molecular mass to the empirical formula mass indicates how many empirical formula units are in one molecule.

30. Can molar mass be used to predict the physical state of a substance?

While molar mass alone cannot definitively predict a substance's physical state, it can provide clues. Generally, substances with very low molar masses (like hydrogen or helium) tend to be gases at room temperature, while those with very high molar masses are more likely to be solids or liquids.

31. How does molar mass relate to the concept of mole fraction?

Molar mass is used to convert between mass and moles when calculating mole fractions. The mole fraction is the ratio of moles of one component to the total moles in a mixture, and molar mass is essential for determining the number of moles from given masses.

32. Why is it important to use the correct molar mass in calculations?

Using the correct molar mass is crucial for accurate calculations in chemistry. Even small errors in molar mass can lead to significant discrepancies in stoichiometric calculations, solution preparations, and other chemical analyses, potentially affecting experimental results and interpretations.

33. How does the concept of average molar mass apply to mixtures?

For mixtures, we often use the concept of average molar mass. This is calculated by taking the weighted average of the molar masses of all components in the mixture, based on their relative abundances. This is particularly useful in gas mixture calculations and polymer chemistry.

34. Can molar mass be used to predict chemical reactivity?

While molar mass alone doesn't determine reactivity, it can provide insights. Smaller molecules (lower molar mass) often have higher mobility and can lead to faster reaction rates. However, reactivity depends on many factors including molecular structure, bond strengths, and electronic configuration.

35. How does molar mass relate to the concept of molality?

Molar mass is essential in calculating molality, which is defined as the number of moles of solute per kilogram of solvent. To determine the number of moles from a given mass of solute, you need to use its molar mass.

36. Why is molar mass important in gas chromatography?

In gas chromatography, molar mass affects the retention time of compounds. Generally, molecules with higher molar masses have longer retention times as they interact more strongly with the stationary phase. This relationship helps in identifying unknown compounds based on their retention times.

37. How does molar mass affect diffusion rates?

Molar mass is inversely related to diffusion rates. According to Graham's law of diffusion, the rate of diffusion is inversely proportional to the square root of the molar mass. This means that molecules with lower molar masses generally diffuse faster than those with higher molar masses.

38. Can molar mass be used to predict boiling points?

While molar mass is not the only factor, it often correlates with boiling point. Generally, substances with higher molar masses tend to have higher boiling points due to stronger intermolecular forces. However, other factors like molecular shape and polarity also play significant roles.

39. How does molar mass relate to the concept of equivalent weight?

Molar mass is used to calculate equivalent weight, which is the mass of a substance that will react with or replace one mole of hydrogen ions in an acid-base reaction or one mole of electrons in a redox reaction. The equivalent weight is often expressed as the molar mass divided by the valence or oxidation state change.

40. Why is understanding molar mass important for environmental scientists?

Environmental scientists often deal with gas laws and solution concentrations when studying air and water pollution. Molar mass is crucial for converting between mass and moles, calculating gas volumes, and determining concentrations of pollutants in various environmental matrices.

41. How does molar mass affect the vapor pressure of a liquid?

Generally, liquids with lower molar masses tend to have higher vapor pressures at a given temperature. This is because lighter molecules typically have weaker intermolecular forces and can more easily escape from the liquid phase into the gas phase.

42. How does molar mass relate to the concept of normality?

Molar mass is used to calculate normality, which is the number of equivalents of solute per liter of solution. To determine the number of equivalents, you need to know the molar mass and the number of reactive species (like H+ ions or electrons) per formula unit of the solute.

43. Why is molar mass important in forensic science?

In forensic science, molar mass is crucial for quantitative analysis of evidence. It's used in techniques like mass spectrometry to identify unknown substances, in toxicology to calculate drug concentrations, and in trace evidence analysis to determine the composition of materials.

44. How does molar mass affect the freezing point of a solution?

Molar mass is inversely related to the freezing point depression of a solution. For a given mass of solute, substances with lower molar masses will cause a greater depression of the freezing point because they contribute more particles to the solution.

45. Can molar mass be used to predict the strength of intermolecular forces?

While molar mass alone doesn't determine the strength of intermolecular forces, it can provide some insights. Generally, molecules with higher molar masses tend to have stronger van der Waals forces. However, other factors like polarity and hydrogen bonding capability also play significant roles.

46. How does molar mass relate to the concept of molarity?

Molar mass is essential for calculating molarity, which is the number of moles of solute per liter of solution. To determine the number of moles from a given mass of solute, you need to use its molar mass. This is crucial for preparing solutions of specific concentrations.

47. Why is understanding molar mass important for pharmacologists?

Pharmacologists use molar mass to calculate drug dosages, determine drug concentrations in the body, and study drug metabolism. It's crucial for converting between mass and moles, which is necessary for understanding drug kinetics and dynamics.

48. How does molar mass affect the rate of effusion of gases?

According to Graham's law of effusion, the rate of effusion of a gas is inversely proportional to the square root of its molar mass. This means that gases with lower molar masses effuse more quickly through small openings than gases with higher molar masses.

49. How does molar mass relate to the concept of mass spectrometry?

In mass spectrometry, molar mass is crucial for identifying compounds. The mass-to-charge ratio (m/z) measured by the instrument is related to the molar mass of the molecule or its fragments. By analyzing these ratios, scientists can determine the molar mass and structure of unknown compounds.

50. Why is molar mass important in the study of reaction kinetics?

In reaction kinetics, molar mass is used to convert between concentration units and to understand how molecular size affects reaction rates. It's also important in interpreting the order of reactions and in calculating rate constants.

51. How does molar mass affect the properties of polymers?

In polymer chemistry, the average molar mass of a polymer significantly affects its properties. Higher molar mass polymers generally have higher melting points, higher viscosity in solution, and better mechanical properties. Understanding molar mass distribution is crucial for predicting polymer behavior.

52. Can molar mass be used to predict the hardness of a solid?

While molar mass alone doesn't determine hardness, it can provide some insights. In some cases, higher molar mass can correlate with increased hardness due to stronger intermolecular forces. However, factors like crystal structure and bond types play more significant roles in determining hardness.

53. Why is understanding molar mass important for atmospheric scientists?

Atmospheric scientists use molar mass in studying gas laws, calculating air density, and understanding the behavior of different atmospheric gases. It's crucial for modeling atmospheric processes, studying greenhouse gases, and analyzing air pollution.

54. How does molar mass affect the specific heat capacity of a substance?

Generally, substances with lower molar masses tend to have higher specific heat capacities. This is because lighter atoms or molecules can typically store more energy in their vibrations and rotations for a given temperature increase. However, other factors like bond strength also play a role.