Question

If 30 ml of H2 and 20 ml of O2 react to form water. What is left at the end of the reaction.

Answer 1

1ml of H₂ reacts with 0.5ml of O₂ to form 1ml of water.
30 ml of H₂ will react with 15 ml of O₂ to form  water. So at the end of the reaction, 30 ml of water and 5 ml of O₂ will be left.

Answer 2

$\mathrm{H}_{2}\ and\ \mathrm{O}_{2}$ react to form water.

The equation is given below:

$\bold{2 \mathrm{H}_{2}+\mathrm{O}_{2} \rightarrow 2 \mathrm{H}_{2} \mathrm{O}}$

Hence 2 moles of $\mathrm{H}_{2}$ react with 1 mole of $\mathrm{O}_{2}$ to form 2 moles of $\mathrm{H}_{2} \mathrm{O}$. We can also say 20 mL of $\mathrm{H}_{2}$ react with 10 mL of $\mathrm{O}_{2}$ to form 20 mL of $\mathrm{H}_{2} \mathrm{O}$. So ‘30 mL’ of $\mathrm{H}_{2}$ will react with ‘15 mL’ of $\mathrm{O}_{2}$.  

Therefore 20 - 15 = 5 mL of $\mathrm{O}_{2}$ will left as excess of reactant whereas 30 mL $\mathrm{H}_{2}$ will form 30 mL of $\mathrm{H}_{2} \mathrm{O}$.

Overall; 5 mL $\mathrm{O}_{2}$ and 30 mL $\mathrm{H}_{2} \mathrm{O}$ will left at the “end of the reaction”.