Find the molecular mass of carbon dioxide, CO₂, and of laughing gas, N₂O.

Step 1. (a) $\text{CO}_2$: $12\ \text{u} + 2 \times 16\ \text{u} = 44\ \text{u}$. $\text{N}_2\text{O}$: $2 \times 14\ \text{u} + 16\ \text{u} = 44\ \text{u}$. They are equal.

Step 2. (b) $\text{N}_2$: $2 \times 14\ \text{u} = 28\ \text{u}$. $\text{CO}$: $12\ \text{u} + 16\ \text{u} = 28\ \text{u}$. $\text{C}_2\text{H}_4$: $2 \times 12\ \text{u} + 4 \times 1\ \text{u} = 28\ \text{u}$.