Two oxides of a metal contain $50\%$ and $40\%$ of the metal,respectively. The formula of the first oxide is $MO$. What is the formula of the second oxide?

Determine the molecular formula of an oxide of iron in which the mass per cent of iron and oxygen are $69.9$ and $30.1$ respectively. Given that the molar mass of the oxide is $159.69 \, g \, mol^{-1}$.

Calculate the mass of $Ca$ in $g$ present in $Ca(NO_3)_2$ that contains $20 \ g$ of nitrogen.

Insulin contains $3.4\%$ sulfur. What is the minimum molecular weight of insulin?

The molar mass of caffeine is $194$. It contains $28.9\%$ of nitrogen by mass. The number of nitrogen atoms in one molecule of caffeine is:

$A$ compound contains $4.07\%$ hydrogen,$24.27\%$ carbon,and $71.65\%$ chlorine. Its molar mass is $98.96\ g/mol$. What are its empirical and molecular formulas?