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

On analysis,a certain compound was found to contain iodine and oxygen in the ratio of $254 \ g$ of iodine and $80 \ g$ of oxygen. The atomic mass of iodine is $127$ and that of oxygen is $16$. Which of the following is the formula of the 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 ...............

The chemical formula of rust is:

An alkane has a $C/H$ ratio (by mass) of $5.1428$. Its molecular formula is

How many grams of oxygen are present in $32.2 \ g$ of $Na_2SO_4 \cdot 10H_2O$ (in $g$)?