Air contains $23\%$ oxygen and $77\%$ nitrogen by weight. What is the percentage by volume of $O_2$?

What fraction of $Fe$ exists as $Fe(III)$ in $Fe_{0.96}O$?
(Consider $Fe_{0.96}$ to be made up of $Fe(II)$ and $Fe(III)$ only)

$0.63 \ g$ of oxalic acid is dissolved to obtain $250 \ cm^3$ of its solution. Find the normality of this solution. [Oxalic acid: $(COOH)_2 \cdot 2H_2O$] (in $N$)

$0.5 \ L$ of gaseous hydrocarbon when burnt in excess of $O_2$ gave $3.5 \ L$ of $CO_2$ and $4 \ L$ of water vapours under same conditions. The molecular formula of the hydrocarbon will be:

$1.86 \ g$ of aniline completely reacts to form acetanilide. $10 \ \%$ of the product is lost during purification. Amount of acetanilide obtained after purification (in $g$) is ...... $\times 10^{-2}$.

$3 \ L$ mixture of $C_2H_6$ and $C_3H_8$ produces $8 \ L$ of $CO_2$ upon complete combustion. Find the volume of $C_2H_6$ in the mixture.