The equation of state of some gases can be expressed as;
$\left(\mathrm{P}+\frac{\mathrm{a}}{{\mathrm{v}}^2}\right)\left(\mathrm{V}-\mathrm{b}\right)=\mathrm{RT}$
where P is the pressure, V the volume, T the absolute temperature and a, b, R are constants. The dimensions of 'a' are: