In a substance,the number of atoms of $Cr$ and $O$ are $4.8 \times 10^{10}$ and $9.6 \times 10^{10}$ respectively. The empirical formula of the substance is .......

$A$ compound containing $Xe$ and $F$ has $63.8\%$ of $Xe$ by mass. Calculate the oxidation number of $Xe$ in this compound.

The molecular mass of caffeine is $194 \, a.m.u.$. It contains $28.9 \%$ of nitrogen by mass. The number of nitrogen atoms in one molecule of caffeine is ...............

Chlorophyll,the green pigment in plants responsible for photosynthesis,contains $2.68\%$ of magnesium by mass. Calculate the number of magnesium atoms in $2.00 \ g$ of chlorophyll.

On combustion,$0.210 \ g$ of an organic compound containing $C$,$H$,and $O$ gave $0.127 \ g$ $H_2O$ and $0.307 \ g$ $CO_2$. The percentages of hydrogen and oxygen in the given organic compound respectively are $:$

An organic compound contains $C = 40\%$,$H = 13.33\%$,and $N = 46.67\%$. Its empirical formula is: