Introduction
Before you can do any real "chemical maths," you need one essential tool: molar mass. It's the bridge that lets you convert between the mass of a substance you can weigh on a balance and the number of particles (atoms, molecules, or ions) it contains, measured in moles. Every stoichiometry calculation in this unit — mass–mass, mass–volume, and concentration problems — starts with this conversion (Talbot, p.424).
What is molar mass?
Molar mass (M) is the mass of one mole of a substance, expressed in grams per mole (g mol⁻¹). Numerically, it is equal to the relative atomic mass (for elements) or the relative formula mass (for compounds) — you just add the unit g mol⁻¹.
- For an element, look up the relative atomic mass (Aᵣ) on the periodic table.
- Example: Aluminium, Al → M = 26.98 g mol⁻¹ (Talbot, p.425)
- For a compound, add together the atomic masses of every atom in the formula. This gives the relative formula mass (Mᵣ), and again, adding g mol⁻¹ gives the molar mass (Talbot, p.86).
The key equation
The relationship between mass, amount (moles), and molar mass is:
n = m / M
where:
- n = amount of substance (mol)
- m = mass (g)
- M = molar mass (g mol⁻¹)
Rearranged forms are just as useful:
m = n × M
M = m / n
Worked Example 1 – Finding molar mass of a compound
Calculate the molar mass of magnesium oxide, MgO.
- Mg: 24.31 g mol⁻¹
- O: 16.00 g mol⁻¹
M(MgO) = 24.31 + 16.00 = 40.31 g mol⁻¹
Worked Example 2 – Finding molar mass of a compound with brackets
Calculate the molar mass of Ca(OH)₂.
- Ca: 40.08
- O: 16.00 × 2 = 32.00
- H: 1.01 × 2 = 2.02
M = 40.08 + 32.00 + 2.02 = 74.10 g mol⁻¹
Remember: anything inside brackets is multiplied by the subscript outside the brackets.
Worked Example 3 – Using n = m / M
How many moles are in 1.64 g of aluminium?
M(Al) = 26.98 g mol⁻¹