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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.
How to Calculate Molar Mass
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, H_{2}O
The relative atomic masses of the elements in water, H_{2}O 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, H_{2}O 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, H_{2}SO_{4}
The relative atomic masses of the elements in H_{2}SO_{4} are: H = 1, S = 32, O = 16.
Calculating the molar mass, we have: 1x2 + 32 + 16x4
= 2 + 32 + 64 = 98
The molar mass of H_{2}SO_{4} is 98g/mol
4. Glucose, C_{6}H_{12}O_{6
}
The relative atomic masses of elements in C_{6}H_{12}O_{6
} 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), CaCO_{3}
The relative atomic masses of elements in CaCO_{3} 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 CaCO_{3} is 100g/mol
6. Sucrose, C_{12}H_{2}2O_{11}
The relative atomic masses of elements in C_{12}H_{22}O_{11} 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, CH_{4}
The relative atomic masses of the elements in CH_{4} 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, CH_{3}COOH
The relative atomic masses of elements in CH_{3}COOH: C = 12, H = 1, O = 16
Calculating the molar mass of CH_{3}COOH, 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, NH_{3}
The relative atomic masses of the elements in NH_{3} are: N = 14, H = 1
Calculating its molar mass: 14 + 1x3
= 14 + 3 = 17
Therefore, the molar mass of NH_{3} is 17g/mol
11. Butane, C_{4}H_{10}
The relative atomic masses of elements in C_{4}H_{10} 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, CuSO_{4}
The relative atomic masses of the elements in CuSO_{4} 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 CuSO_{4} 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, CaCl_{2}
Relative atomic masses of elements in CaCl_{2} are: Ca = 40, Cl = 35.5
Calculating its molar mass, we have, 40 + 35.5x2
= 40 + 71 = 111
Therefore, the molar mass of CaCl_{2} is 111g/mol
15. Methanol, CH_{3}OH
Relative atomic masses of elements in CH_{3}OH 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, C_{2}H_{5}OH
The relative atomic masses of elements in C_{2}H_{5}OH 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, CO_{2}
The relative atomic masses of the elements in CO_{2}: 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, C_{3}H_{6}O
The elements in C_{3}H_{6}O 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, C_{7}H_{6}O_{3}
The elements in C_{7}H_{6}O_{3} 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, C_{6}H_{6}
The elements in C_{6}H_{6} 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, C_{9}H_{8}O_{4}
The relative atomic masses of elements in C_{9}H_{8}O_{4} 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
How to Determine the Molar Mass of Air
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 106 or 0.09 x 104 %).
Calculating the molar mass of each gas in air:
Oxygen, O_{2}, molecular mass = 16 x 2 = 32. Molar mass of oxygen in air = 32 x 0.2095 = 6.704g/mol
Nitrogen, N_{2}, molecular mass = 14 x 2 = 28. Molar mass of nitrogen in air = 28 x 0.7809 = 21.88g/mol
Carbon dioxide, CO_{2}, molecular mass = 12 + 16x2 = 12 + 32 = 44. Molar mass of carbon dioxide in air = 44 x 0.0003 = 0.0132g/mol
Hydrogen, H_{2}, 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 didnâ€™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 Diatomic or Polyatomic Molecule
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, N_{2}
The relative atomic mass of nitrogen is 14. The molar mass of nitrogen molecule, N_{2} is 14 x 2 = 28g/mol
2. Oxygen, O_{2}
The relative atomic mass of Oxygen is 16, therefore, the molar mass of oxygen,
O_{2} is 16 x 2 = 32g/mol
3. Chlorine, Cl_{2}
The relative atomic mass of chlorine is 35.5. The molar mass of chlorine
molecule Cl_{2} is 35.5 x 2 = 71g/mol
4. Hydrogen, H_{2}
The relative atomic mass of hydrogen is 1, therefore, its molar mass is 1 x 2 = 2g/mol
5. Sulphur, S_{8}
The relative atomic mass of sulphur is 32, therefore, the molar mass of S_{8} is 32 x 8 = 256g/mol.
