$1.0 \mathrm{g}$ of hydrogen contains $6\times {10}^{23}$ atoms. The atomic weight of helium is $4$. It follows that the number of atoms in $1 \mathrm{g}$ of $\mathrm{He}$ is:

A sample of aluminium has a mass of $54.0 \mathrm{g}$. What is the mass of the same number of magnesium atoms?

$($At. wt.$\mathrm{Al}=27, \mathrm{Mg}=24)$

The volume of ${\mathrm{CO}}_2$ obtained at $\mathrm{STP}$ by the complete decomposition of $9.85 \mathrm{g} \mathrm{of} {\mathrm{BaCO}}_3$, is

(Molar mass of $\mathrm{Ba}=137 \mathrm{g}/\mathrm{mol}$ )

The volume of a gas at $\mathrm{NTP}$ produced by $100\mathrm{g}$ of ${\mathrm{CaC}}_2$ with water:

${\mathrm{CaC}}_2+{\mathrm{H}}_2\mathrm{O}⟶\mathrm{Ca}(\mathrm{OH}{)}_2+{\mathrm{C}}_2{\mathrm{H}}_2$