An oxide of iron,which contains $69.9 \%$ iron and $30.1 \%$ dioxygen by mass,has the empirical formula:

The complete combustion of $0.492 \, g$ of an organic compound containing $'C'$,$'H'$,and $'O'$ gives $0.793 \, g$ of $CO_{2}$ and $0.442 \, g$ of $H_{2}O$. The percentage of oxygen composition in the organic compound is .... (nearest integer)

Number of $g$ of oxygen in $32.2\,g$ of $Na_2SO_4 \cdot 10H_2O$ is

Haemoglobin contains $0.33 \%$ of iron by mass. The molecular mass of haemoglobin is $67200$. The number of iron atoms present in one molecule of haemoglobin is: (Atomic mass of $Fe = 56$)

The chemical formula of rust is:

Determine the empirical formula of Kevlar,used in making bulletproof vests,which contains $70.6 \% \, C, \, 4.2 \% \, H, \, 11.8 \% \, N$ and $13.4 \% \, O$.