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

The empirical formula of a compound is $CH_2O$ and its vapor density is $30$. The molecular formula of the compound is ......

What is the percentage by mass of oxygen in $NaOH$? (Atomic mass of $Na=23 \ u, O=16 \ u, H=1 \ u$)

How much copper can be obtained from $100 \, g$ of copper sulphate $(CuSO_{4})$?

The elemental composition of a compound is $54.2 \% C$,$9.2 \% H$,and $36.6 \% O$. If the molar mass of the compound is $132 \ g \ mol^{-1}$,the molecular formula of the compound is: [Given: The relative atomic mass of $C:H:O = 12:1:16$]

When a crystalline salt $Na_2SO_4 \cdot xH_2O$ is heated,it loses $55.9\%$ of its weight. What is the formula of the crystalline salt?