We know that: P_{ideal} = P_{real }+ an^{2}/V^{2}

P_{ideal} – P_{real}= an^{2}/V^{2}

Nm^{-2} = a*mol^{2}/m^{6}

A = Nm^{4}mol^{-2}

The unit of ‘a’ when the pressure is taken in Nm^{-2}, number of moles in “mol” and volume in m^{3} is Nm^{4}mol^{-2}

when pressure is in atmosphere and volume in dm^{3} then, the value of ‘a’ is:

a = atm x mol^{2} x dm^{-6}