Home

Molar Mass

The molar mass of a substance (chemical element or compound) is the mass of one mole of the substance. It is a physical quantity that is measured and expressed in g/mol.

The molar mass of a substance is calculated by summing up the product of the relative atomic mass and number of atoms of all the elements that make up the substance as expressed in its chemical formula. For an element, the molar mass is the atomic mass of the element.

For example: calculate the molar mass of the following substances:

1. Water, H₂O

The relative atomic masses of the elements in water, H₂O are: H = 1, O = 16.

Therefore, the molar mass is: relative atomic mass of hydrogen x number of atoms of hydrogen + relative atomic mass of oxygen x number of atoms of hydrogen.

This is expressed as 1x2 + 16x1 = 2 + 16 = 18

The molar mass of water, H₂O is therefore 18g/mol

2. Hydrochloric acid, HCl

The relative atomic masses of the elements in hydrochloric acid, HCl are H = 1, Cl = 35.5.

The molar mass is calculated as follows: relative atomic mass of H + relative atomic mass of chlorine, which is 1 + 35.5 = 36.5g/mol

3. Tetraoxosulphate(VI) acid, H₂SO₄

The relative atomic masses of the elements in H₂SO₄ are: H = 1, S = 32, O = 16.

Calculating the molar mass, we have: 1x2 + 32 + 16x4

= 2 + 32 + 64 = 98

The molar mass of H₂SO₄ is 98g/mol

4. Glucose, C₆H₁₂O₆

The relative atomic masses of elements in C₆H₁₂O₆  are: C = 12, H = 1, O = 16.

Calculating the molar mass, we have: 12x6 + 1x12 + 16x6

= 72 + 12 + 96 = 180

Therefore, the molar mass of glucose is 180g/mol

5. Calcium trioxocarbonate(IV), CaCO₃

The relative atomic masses of elements in CaCO₃ are Ca = 40, C = 12, O = 16

Calculating the molar mass, we have: 40 + 12 + 16x3

= 40 + 12 + 48 = 100

Therefore, the molar mass of CaCO₃ is 100g/mol

6. Sucrose, C₁₂H₂2O₁₁

The relative atomic masses of elements in C₁₂H₂₂O₁₁ are: C = 12, H = 1, O = 16

Calculating the molar mass, we have 12x12 + 1x22 + 16x11

= 144 + 22 + 176 = 342

Therefore, the molar mass of sucrose is 342g/mol

7. Methane, CH₄

The relative atomic masses of the elements in CH₄ are: C = 12, H = 1

Calculating the molar mass gives 12 + 1x4

= 12 + 4 = 16

The molar mass of methane is 16g/mol

8. Acetic acid, CH₃COOH

The relative atomic masses of elements in CH₃COOH: C = 12, H = 1, O = 16

Calculating the molar mass of CH₃COOH, we have 12x2 + 1x4 + 16x2

= 24 + 4 + 32 = 60

Therefore, the molar mass of acetic acid is 60g/mol

9. Sodium hydroxide, NaOH

The relative atomic masses of the elements in NaOH are: Na = 23, O = 16, H = 1

Calculating the molar mass of NaOH: 23 + 16 + 1 = 40

Therefore, the molar mass of sodium hydroxide is 40g/mol

10. Ammonia, NH₃ The relative atomic masses of the elements in NH₃ are: N = 14, H = 1

Calculating its molar mass: 14 + 1x3

= 14 + 3 = 17

Therefore, the molar mass of NH₃ is 17g/mol

11. Butane, C₄H₁₀ The relative atomic masses of elements in C₄H₁₀ are: C = 12, H = 1

Calculating its molar mass: 12x4 + 1x10

= 48 + 10 = 58

Therefore, the molar mass of butane is 58g/mol

12. Copper(II) sulphate, CuSO₄ The relative atomic masses of the elements in CuSO₄ are: Cu = 64, S = 32, O = 16

Calculating the molar mass, we have: 64 + 32 + 16x4

= 64 + 32 + 64 = 160

Therefore, the molar mass of CuSO₄ is 160g/mol

13. Potassium chloride, KCl

The relative atomic masses of elements in KCl are: K = 39, Cl = 35.5

Calculating its molar mass, 39 + 35.5 = 74.5

The molar mass of KCl is therefore 74.5g/mol

14. Calcium chloride, CaCl₂ Relative atomic masses of elements in CaCl₂ are: Ca = 40, Cl = 35.5

Calculating its molar mass, we have, 40 + 35.5x2

= 40 + 71 = 111

Therefore, the molar mass of CaCl₂ is 111g/mol

15. Methanol, CH₃OH

Relative atomic masses of elements in CH₃OH are: C = 12, H = 1, O = 16

Calculating the molar mass, we have, 12 + 1x4 + 16

= 12 + 4 + 16 = 32

The molar mass of methanol is 32g/mol

16. Ethanol, C₂H₅OH

The relative atomic masses of elements in C₂H₅OH are: C = 12, H = 1, O = 16

Calculating its molar mass = 12x2 + 1x6 + 16

= 24 + 6 + 16 = 46

The molar mass of ethanol is therefore 46g/mol

17. Carbon(IV) oxide, also called carbon dioxide, CO₂

The relative atomic masses of the elements in CO₂: C = 12, O = 16

Calculating its molar mass, we have 12 + 16x2

= 12 + 32 = 44

Therefore, the molar mass of carbon(IV) oxide is 44g/mol

18. Sodium chloride, NaCl

The relative atomic masses of elements in NaCl: Na = 23, Cl = 35.5

Calculating its molar mass, we have 23 + 35.5 = 58.5

The molar mass of NaCl is 58.5g/mol

19. Acetone, C₃H₆O The elements in C₃H₆O and their relative atomic masses are: C = 12, H = 1, O = 16

Calculating its molar mass: 12x3 + 1x6 + 16

= 36 + 6 + 16 = 58

Therefore, the molar mass of acetone is 58g/mol

20. Salicylic acid, C₇H₆O₃

The elements in C₇H₆O₃ and their relative atomic masses are: C = 12, H = 1, O = 16

Calculating its molar mass: 12x7 + 1x6 + 16x3

= 84 + 6 + 48 = 138

Therefore, the molar mass of salicylic acid is 138g/mol

21. Benzene, C₆H₆ The elements in C₆H₆ and their relative atomic masses are: C = 12, H = 1

Calculating the molar mass: 12x6 + 1x6

= 72 + 6 = 78

The molar mass of benzene is therefore 78g/mol

22. Aspirin, C₉H₈O₄

The relative atomic masses of elements in C₉H₈O₄ are: C = 12, H = 1, O = 16

Calculating its molar mass: 12x9 + 1x8 + 16x4

= 108 + 8 + 64 = 180

Therefore, the molar mass of aspirin is 180g/mol

Air is a mixture of gases; therefore, its molar mass can be determined by summing up the molar masses of all the constituent gases.

The gases that make up air are: oxygen, nitrogen, carbon dioxide (or carbon(IV) oxide), hydrogen, argon, neon, helium, krypton, xenon. The actual molar mass of each gas present in the mixture can be found out by multiplying the mass of one molecule of the gas by its volume ratio to dry air (or the percentage of the gas in dry air).

The volume ratio of the gases to dry air (or the percentage of the gases in dry air) are given as: oxygen (0.2095 or 20.95%); nitrogen (0.7809 or 78.09%); carbon dioxide (0.0003 or 0.03%); hydrogen (0.0000005 or 0.00005%); argon (0.00933 or 0.933%); neon (0.000018 or 0.0018%); helium (0.000005 or 0.0005%); krypton (0.000001 or 0.0001%); xenon (0.09 x 10-6 or 0.09 x 10-4 %).

Calculating the molar mass of each gas in air: Oxygen, O₂, molecular mass = 16 x 2 = 32. Molar mass of oxygen in air = 32 x 0.2095 = 6.704g/mol

Nitrogen, N₂, molecular mass = 14 x 2 = 28. Molar mass of nitrogen in air = 28 x 0.7809 = 21.88g/mol

Carbon dioxide, CO₂, molecular mass = 12 + 16x2 = 12 + 32 = 44. Molar mass of carbon dioxide in air = 44 x 0.0003 = 0.0132g/mol

Hydrogen, H₂, molecular mass = 1 x 2 = 2. Molar mass of hydrogen in air = 2 x 0.0000005 = 0.000001g/mol

Argon, Ar, atomic mass = 40. Molar mass of argon in air = 40 x 0.00933 = 0.3732g/mol

Neon, Ne, atomic mass = 20. Molar mass of neon in air = 20 x 0.000018 = 0.00036g/mol

Helium, He, atomic mass = 4. Molar mass of helium in air = 4 x 0.000005 = 0.00002g/mol

Krypton, Kr, atomic mass = 84. Molar mass of krypton in air = 84 x 0.000001 = 0.000084g/mol

Xenon, Xe, atomic mass = 131. Molar mass of xenon in air = 131 x 0.09 x 10^-6 = 0.1179 x 10^-4

Molar mass of dry air = 6.704 + 21.88 + 0.0132 + 0.3732 = 28.97g/mol

Notice that we didn’t include the molar masses of hydrogen, neon, helium, krypton, and xenon because they are too small to significantly affect the overall molar mass of air.

The molar mass of a diatomic or polyatomic molecule is same as the atomic mass of its element multiplied by the number of atoms contained.

Examples: Determine the molar mass of the following:

1. Nitrogen, N₂

The relative atomic mass of nitrogen is 14. The molar mass of nitrogen molecule, N₂ is 14 x 2 = 28g/mol

2. Oxygen, O₂

The relative atomic mass of Oxygen is 16, therefore, the molar mass of oxygen, O₂ is 16 x 2 = 32g/mol

3. Chlorine, Cl₂

The relative atomic mass of chlorine is 35.5. The molar mass of chlorine molecule Cl₂ is 35.5 x 2 = 71g/mol

4. Hydrogen, H₂

The relative atomic mass of hydrogen is 1, therefore, its molar mass is 1 x 2 = 2g/mol

5. Sulphur, S₈ The relative atomic mass of sulphur is 32, therefore, the molar mass of S₈ is 32 x 8 = 256g/mol.