8 g of hydrogen burns in oxygen: 2H₂ + O₂ → 2H₂O.

Step 1. Moles. $\dfrac{8\ \text{g}}{2\ \text{g/mol}} = 4\ \text{mol}$ of $\text{H}_2$.

Step 2. Water. $4\ \text{mol} \times 18\ \text{g/mol} = 72\ \text{g}$.

Step 3. Oxygen. $2\ \text{mol} \times 32\ \text{g/mol} = 64\ \text{g}$.