0.5 g of sample of an alloy gave 0.30 g of ${\mathrm{Mg}}_2{\mathrm{P}}_2{\mathrm{O}}_7$ (molar mass of Mg and P are  $24\mathrm{g}{\mathrm{mol}}^{-1}$ and $31\mathrm{g}{\mathrm{mol}}^{-1}\text{, }$respectively). The percentage of Mg in the alloy would be

Molar mass of ${\mathrm{Mg}}_2{\mathrm{P}}_2{\mathrm{O}}_7=(2\times 24+2\times 31+7\times 16)\mathrm{g}{\mathrm{mol}}^{-1}=222\mathrm{g}{\mathrm{mol}}^{-1}$Mass of Mg in the given  ${\mathrm{Mg}}_2{\mathrm{P}}_2{\mathrm{O}}_7=\frac{2\times 24}{222}\times 0.30\mathrm{g}=0.06486\mathrm{g}$

Mass percentage of Mg in alloy $=\frac{0.06486\mathrm{g}}{05\mathrm{g}}\times 100=12.97\mathrm{\%}$