What is the mass percent of oxygen in ethanol $(C_2H_5OH)$ (in $\%$)?

$A$ $1000 \ g$ sample of $NaOH$ contains $3 \ moles$ of $O$ atoms. What is the $Purity$ of $NaOH$ $\dots \dots \dots \dots \dots \dots \%$

$A$ $400 \ mg$ iron capsule contains $100 \ mg$ of ferrous fumarate,$(C_2H_2O_4)Fe$. The percentage of iron present in it is approximately $.......... \%$.

When $0.01 \ mol$ of an organic compound containing $60 \%$ carbon was burnt completely,$4.4 \ g$ of $CO_2$ was produced. The molar mass of the compound is $......... \ g \ mol^{-1}$ (Nearest integer).

The molecular formula of sulphur is

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: