The percentage of nitrogen by mass in ammonium sulphate is closest to $.....$ $\%$ (atomic masses of $H = 1, N = 14, O = 16, S = 32$ ).

$A$ compound $X$ contains $32 \%$ of $A$,$20 \%$ of $B$,and the remaining percentage of $C$. Then,the empirical formula of $X$ is:
(Given atomic masses of $A = 64$,$B = 40$,$C = 32 \ u$)

The empirical formula of a compound is $CH_2O$ and its molecular weight is $120$. The molecular formula of the compound is:

Calculate the mass percentage of different elements present in sodium sulfate $(Na_2SO_4)$.

An ornamental of gold having $75\%$ of gold,it is of .............. carat.

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