Find the formula unit mass of sodium sulphate, Na₂SO₄. Use Na = 23 u, S = 32 u and O = 16 u.

Step 1. Sodium. $2 \times 23\ \text{u} = 46\ \text{u}$.

Step 2. Sulphur and oxygen. $32\ \text{u} + 4 \times 16\ \text{u} = 96\ \text{u}$.

Step 3. Total. $46\ \text{u} + 96\ \text{u} = 142\ \text{u}$.