Complete combustion of a hydrocarbon yields $0.72 \ g$ of water and $3.08 \ g$ of $CO_2$. What is the empirical formula of the hydrocarbon?

$A$ substance contains $38.8\% \ C$,$16.0\% \ H$,and $45.2\% \ N$. What is the empirical formula of the substance?

$A$ carbon compound contains $12.8 \%$ of carbon,$2.1 \%$ of hydrogen and $85.1 \%$ of bromine. The molecular weight of the compound is $187.9$. Calculate the molecular formula of the compound. (Atomic weight: $H=1.008, C=12.0, Br=79.9$)

In a compound,the ratio of masses of $H, C, O,$ and $N$ is $1:3:4:7$. The empirical formula is

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:

The molecular mass of an organic monobasic acid is $129$ and the value of $n$ is $2$. What is the empirical formula mass of the compound?