510 mg of liquid on vaporization in Victor Meyer's apparatus displaces $67.2{\mathrm{cm}}^3$ of air at (STP). The molecular weight of the liquid is _____

Number of moles of air displaced by liquid = $\frac{\mathrm{vol} \mathrm{of} \mathrm{air} \mathrm{displaced}}{22.4\mathrm{lit}.}=\frac{67.2\times {10}^{-3}}{22.4}=3\times {10}^{-3}$moles

Molecular weight of  liquid = $\frac{\mathrm{Wt}.}{\mathrm{no}.\mathrm{of} \mathrm{moles}}$$=\frac{510\times {10}^{-3}\mathrm{g}}{3\times {10}^{-3}}=170\mathrm{g}$   (since n=$\frac{\mathrm{wt}.}{\mathrm{mol}.\mathrm{wt}.}$)