A flask of volume $120 {\mathrm{cm}}^3$ is filled with helium gas at r.t.p. Calculate the mass of helium present in the flask. (${\mathrm{A}}_{\mathrm{r}}$ value: $\mathrm{He}=4.0$). 

$50 {\mathrm{cm}}^3$ of a gaseous hydride of phosphorus, ${\mathrm{PH}}_{\mathrm{n}}$ reacts with exactly $150 {\mathrm{cm}}^3$ of chlorine, ${\mathrm{Cl}}_2$, to form liquid phosphorous trichloride and $150 {\mathrm{cm}}^3$ of hydrogen chloride gas, $\mathrm{HCl}$. How many moles of chlorine react with $1$ mole of the gaseous hydride?

$50 {\mathrm{cm}}^3$ of a gaseous hydride of phosphorus, ${\mathrm{PH}}_{\mathrm{n}}$ reacts with exactly $150 {\mathrm{cm}}^3$ of chlorine, ${\mathrm{Cl}}_2$, to form liquid phosphorous trichloride and $150 {\mathrm{cm}}^3$ of hydrogen chloride gas, $\mathrm{HCl}$. Deduce the formula of the phosphorus hydride.

$50 {\mathrm{cm}}^3$ of a gaseous hydride of phosphorus, ${\mathrm{PH}}_{\mathrm{n}}$ reacts with exactly $150 {\mathrm{cm}}^3$ of chlorine, ${\mathrm{Cl}}_2$, to form liquid phosphorous trichloride and $150 {\mathrm{cm}}^3$ of hydrogen chloride gas, $\mathrm{HCl}$. Write a balanced equation for the reaction.

What is the volume of carbon dioxide produced when $14 \mathrm{g}$ of ethene, ${\mathrm{C}}_2{\mathrm{H}}_4$, reacts with excess oxygen? (${\mathrm{M}}_{\mathrm{r}}$ ethene$=28$).

${\mathrm{C}}_2{\mathrm{H}}_4\left(\mathrm{g}\right)+3{\mathrm{O}}_2\left(\mathrm{g}\right)\to 2{\mathrm{CO}}_2\left(\mathrm{g}\right)+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$.

Boron is present in compounds called borates. Use the ${\mathrm{A}}_{\mathrm{r}}$ values below to calculate the relative molecular mass of iron(III) borate, $\mathrm{Fe}{\left({\mathrm{BO}}_2\right)}_3$.

(${\mathrm{A}}_{\mathrm{r}}$ values: $\mathrm{Fe}=55.8, \mathrm{B}=10.8, \mathrm{O}=16.0$).