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:

$A$ 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: $H=1.008, C=12.0, Br=79.9$)

$A$ compound contains $2.2 \ g$ of phosphorus and $1.24 \ g$ of sulfur. What is its empirical formula?

$A$ pure compound contains $2.4 \ g$ of $C$,$1.2 \times 10^{23}$ atoms of $H$,and $0.2 \ mol$ of oxygen atoms. Its empirical formula is

The value of $x$ in $KAl(SO_4)_x \cdot 12 H_2O$ is $.....$

Two oxides of a metal $X$ contain $50 \%$ and $40 \%$ of oxygen respectively. If the formula of the first oxide is $XO_2$,then the formula of the second oxide is: