4 g of hydrogen is burnt in 16 g of oxygen according to 2H$_2$ + O$_2$ → 2H$_2$O. What mass of water is formed? (H = 1, O = 16)

Moles: H$_2$ $= \dfrac{4}{2} = 2$; O$_2$ $= \dfrac{16}{32} = 0.5$. Two moles of H$_2$ need 1 mol of O$_2$, but only 0.5 mol is available, so oxygen is the limiting reagent. It produces $2 \times 0.5 = 1$ mol of water $= 18$ g. Assuming all the hydrogen reacts gives 36 g; adding the reactant masses gives 20 g.