Mole calculations

This equation shows how relative formula mass, number of moles and mass are related:

number of moles = mass ÷ relative formula mass

This can be rearranged to find the mass if the number of moles and molar mass (its relative formula mass in grams) are known. It can also be rearranged to find the molar mass if the mass and number of moles are known.

The triangle diagram may help you with this.

Triangle divided into three with mass in top section, moles and below left and molar mass below right, with an 'x' between them

Finding the number of moles

QQuestion: Calculate the number of moles of carbon dioxide molecules in 22 g of CO₂.
A

Finding the mass

QQuestion: Calculate the mass of 2 mol of carbon dioxide (CO₂).

Finding the relative formula mass

QQuestion: 10 mol of carbon dioxide has a mass of 440 g. What is the relative formula mass of carbon dioxide?

Reacting masses

You can calculate the mass of a product or reactant using the idea of moles, a balanced equation and relevant A_r values.

Example

Sulfuric acid and sodium hydroxide react together to make sodium sulfate and water:

H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O

QQuestion: Calculate the mass of sodium sulfate made when 20 g of sodium hydroxide reacts with excess sulfuric acid. (A_r of H = 1, A_r of O = 16, A_r of Na = 23, A_r of S = 32)