What is the volume occupied by $0.5 \ mol$ of $CO_2$ at $STP$ (in $dm^3$)?

If the enthalpy of combustion of anhydrous oxalic acid is $x \, kcal \, mol^{-1}$,then how much heat in $kcal$ will be produced by the combustion of $2 \, g$ of oxalic acid?

$10 \text{ g}$ of a mixture of $BaO$ and $CaO$ requires $100 \text{ cm}^3$ of $2.5 \text{ M } HCl$ to react completely. The percentage of calcium oxide in the mixture is approximately (Given: molar mass of $BaO = 153 \text{ g/mol}$,$CaO = 56 \text{ g/mol}$).

Air contains $23\%$ oxygen and $77\%$ nitrogen by weight. What is the percentage by volume of $O_2$?

The vapor density of a gas is $11.2$. What is the volume occupied by $1 \, g$ of this gas at $STP$ in $L$?

What volume of ammonia is formed when $10 \ dm^3$ dinitrogen reacts with $30 \ dm^3$ dihydrogen at same temperature and pressure (in $dm^3$)?