If two compounds have the same empirical formula but different molecular formulae,they must have:

$A$ compound gave $80\%$ carbon and $20\%$ hydrogen on analysis. The compound is possibly

Determine the empirical formula of an oxide of iron which has $69.9 \%$ iron and $30.1 \%$ dioxygen by mass.

In a compound made of $X$ and $Y$,their masses are equal. If the atomic masses of $X$ and $Y$ are $30$ and $20$ respectively,then the molecular formula of the compound will be ......... (Its molecular mass $= 120$).

An organic compound with $C = 40\%$ and $H = 6.7\%$ will have the empirical formula

An organic compound on analysis gives the following composition: $C = 54.5\%, O = 36.4\%, H = 9.1\%$. The empirical formula of the compound is ..