$A$ hydrocarbon contains $80\% \ C$. The vapour density of the compound is $30$. The molecular formula of the compound is :-

Complete combustion of $X$ g of an organic compound gave $0.25$ g of $CO_2$ and $0.12$ g of $H_2O$. If the percentage of carbon is $25\%$ and of hydrogen is $4.89\%$, then $X = \_\_\_\_ \times 10^{-3}$ g. (Nearest integer) (Molar mass of $C$, $H$ and $O$ are $12, 1$ and $16$ g mol$^{-1}$ respectively.)

Determine the molecular formula of an oxide of iron in which the mass per cent of iron and oxygen are $69.9$ and $30.1$ respectively. Given that the molar mass of the oxide is $159.69 \, g \, mol^{-1}$.

$A$ substance contains $38.8\%$ carbon,$16\%$ hydrogen,and $45.2\%$ nitrogen. What is the empirical formula of this substance?

What is the mass percentage of carbon in urea (in $\%$)? (molar mass of urea $= 60 \ g \ mol^{-1}$)

$A$ certain compound has the molecular formula $X_4O_6$. If $10 \ g$ of $X_4O_6$ has $5.72 \ g$ of $X$,the atomic mass of $X$ is ............ $amu$.