Moles and masses - Higher
Molar mass
The molar mass of a substance is the mass of one moleThe amount of substance that contains the same number of particles as there are atoms in 12 g of carbon-12 (contains the Avogadro's constant 6.0 ×10²³ number of particles). of that substance. It is equal to:
- the relative atomic massThe mean relative mass of the atoms of the different isotopes in an element. It is the number of times heavier an atom is than one-twelfth of a carbon-12 atom. or Ar in grams for atomThe smallest part of an element that can exist. and metalShiny element that is a good conductor of electricity and heat, and which forms basic oxides.
- the relative formula massThe sum of the relative atomic masses of the atoms in a chemical formula. or Mr in grams for moleculeA collection of two or more atoms held together by chemical bonds. and compoundA substance formed by the chemical union of two or more elements.
The table shows some examples.
| Substance | Formula | Ar or Mr | Molar mass (g/mol) |
| Neon | Ne | Ar = 20.2 | 20.2 |
| Magnesium | Mg | Ar = 24.3 | 24.3 |
| Oxygen | O2 | Mr = 2 Ă— 16.0 = 32.0 | 32.0 |
| Calcium carbonate | CaCO3 | Mr = 40.1 + 12.0 + (3 Ă— 16.0) = 100.1 | 100.1 |
| Substance | Neon |
|---|---|
| Formula | Ne |
| Ar or Mr | Ar = 20.2 |
| Molar mass (g/mol) | 20.2 |
| Substance | Magnesium |
|---|---|
| Formula | Mg |
| Ar or Mr | Ar = 24.3 |
| Molar mass (g/mol) | 24.3 |
| Substance | Oxygen |
|---|---|
| Formula | O2 |
| Ar or Mr | Mr = 2 Ă— 16.0 = 32.0 |
| Molar mass (g/mol) | 32.0 |
| Substance | Calcium carbonate |
|---|---|
| Formula | CaCO3 |
| Ar or Mr | Mr = 40.1 + 12.0 + (3 Ă— 16.0) = 100.1 |
| Molar mass (g/mol) | 100.1 |
Masses from moles
The massThe amount of matter an object contains. Mass is measured in kilograms (kg) or grams (g). of a substance in grams can be calculated from its amount in moles and its molar mass:
mass (g) = molar mass (g/mol) Ă— amount (mol)
Example
Calculate the mass of 0.25 mol of carbon dioxide molecules, CO2. (Relative atomic masses: C = 12.0, O = 16.0)
relative formula mass, Mr = 12.0 + (2 Ă— 16.0) = 44.0
molar mass = 44.0 g/mol
= 11.0 g
The calculation is the same if a substance is a metal or exists as separate atoms, but its Ar is used to work out the molar mass instead of an Mr.
Question
Calculate the mass of 0.10 mol of iron. (Relative atomic mass: Fe = 55.8)
molar mass = 55.8 g/mol
mass = 55.8 Ă— 0.10
= 5.58 g
Moles from masses
The amount of a substance in moles can be calculated from its mass in grams and its molar mass:
\(amount\ (mol)\ = \frac{mass (g)}{molar\ mass\ (g/mol)}\)
Example
Calculate the amount of carbon atoms in 6.0 g of carbon. (Relative atomic mass: C = 12.0)
molar mass = 12.0 g/mol
\(amount\ =\ \frac{6.0}{12.0}\)
= 0.50 mol
Question
Calculate the amount of water molecules in 22.5 g of water, H2O. (Relative atomic masses: H = 1.0, O = 16.0)
relative formula mass, Mr = (2 Ă— 1.0) + 16.0 = 18.0
molar mass = 18.0 g/mol
\(amount\ =\ \frac{22.5}{18.0}\)
= 1.25 mol