The Van der Waal's equation of state for some gases can be expressed as:

$(P + \dfrac{a}{V^2})(V - b) = RT$

where P is the pressure, V is the molar volume, and T is the absolute temperature of the given sample of gas and a,b and R are constants.

The dimensions of a are

$\left[ P \right] =\left[ \dfrac { a }{ { V }^{ 2 } } \right]$

$\left[ a \right] =\left[ P \right] \left[ { V }^{ 2 } \right]$

$\left[ a \right] =\left[ \dfrac { Force }{ Area } \times { \left( Volume \right) }^{ 2 } \right]$

$\left[ a \right] =\left[ \dfrac { { MLT }^{ -2 }\times { L }^{ 6 } }{ { L }^{ 2 } } \right]$

$\left[ a \right] =\left[ { ML }^{ 5 }{ T }^{ -2 } \right]$