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

$A$ compound contains $75.8\%$ of $X$ and $24.2\%$ of $Y$ by mass. If the atomic weights of $X$ and $Y$ are $24$ and $16$ respectively,find the empirical formula of the compound.

An organic compound has a molar mass of $108 \ g \ mol^{-1}$. The atoms $C, H,$ and $N$ are present in the weight ratio of $9 : 1 : 3.5$. What is the molecular formula of the compound?

The percentage of nitrogen in urea is about

Empirical formula of a compound is $CH_2O$ and its vapour density is $30$. The molecular formula of the compound is:

On reduction with hydrogen,$3.6 \ g$ of an oxide of metal $(M)$ left $3.2 \ g$ of the metal. If the atomic weight of the metal is $64$,the formula of the oxide is: