5 g each of the following gases at 87^oC and 750 mm pressure are taken. Which of them will have the least volume?

$\mathrm{PV}=\frac{\mathrm{w}}{\mathrm{M}}\mathrm{RT}$
If other factors are same $\mathrm{V}\propto \frac{1}{\mathrm{m}}$

Higher the mass lower will be the volume. 

The order of molecular mass is $\mathrm{HF}<\mathrm{HCl}<\mathrm{HBr}<\mathrm{HI}$

As the molecular mass of HI is higher, HI will have the least volume.