Complete combustion of 750 g of an organic compound provides 420 g of $CO_2 $ and 210 g of $H_2 O $. The percentage composition of carbon and hydrogen in organic compound is 15.3 and _ _ _ _ respectively. (Round off to the nearest integer)

*Solution*

${n_{{H_2}O}} = \frac{{210}}{{18}} = \frac{{35}}{3}$

${n_H} = 2 \times {n_{{H_2}O}} = 2 \times \frac{{35}}{3} = \frac{{70}}{3}$

${m_H} = 1 \times \frac{{70}}{3}g$

% of H $ = \frac{{70 \times 100}}{{3 \times 750}} = 3.11\% \approx 3\% $

Answer: 3