An organic compound has an empirical formula of $CH_2O$ and a molecular mass of $90$. What is its molecular formula? $(C = 12, H = 1, O = 16)$

Caffeine has a molecular weight of $194 \text{ g/mol}$. If it contains $28.9\%$ by mass of nitrogen, the number of atoms of nitrogen in one molecule of caffeine is:

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 $159.69 \, g \, mol^{-1}$.

The percentage composition of $H, C$,and $O$ in $H_2C_2O_4$ is respectively:

$A$ sample of a hydrate of barium chloride weighing $61 \ g$ was heated until all the water of hydration was removed. The dried sample weight is $52 \ g$. The formula of the hydrated salt is: (atomic mass: $Ba = 137 \ amu, Cl = 35.5 \ amu$)

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