4g of hydrogen is burnt with 4g of oxygen.. How many grams of water can be formed?

The balanced chemical reaction for formation of water  is$2{\mathrm{H}}_2 +{\mathrm{O}}_2 \to 2{\mathrm{H}}_2\mathrm{O}$.

2 mol H₂ reacts with 1 mole of O₂  and forms 2 mol of H₂O .

4 g of H will react with 32 g of oxygen. 

But , only 4 g of oxygen are present.

Thus, oxygen is the limiting reagent.

$4 \mathrm{g} \mathrm{of} \mathrm{oxygen} \mathrm{will} \mathrm{give} =\frac{4\times 36}{32}=4.5\mathrm{g} \mathrm{of} \mathrm{water}$