Write the van der Waals equation

For a real gas, the van der Waals equation is $\left(P + \frac{a n^2}{V^2}\right)(V-nb)=nRT$

Identify the meaning of each correction term

• The term $nb$ corrects for the finite volume of gas molecules.
• The term $\dfrac{a n^2}{V^2}$ corrects for intermolecular attractive forces.

Why this pressure correction appears

In a real gas, attractive forces between molecules reduce the observed pressure compared to an ideal gas. So, to correct the pressure, we add $\frac{a n^2}{V^2}$ to the measured pressure $P$.

Check the options

• A: $nb$ → volume correction, not attraction correction.
• B: $\dfrac{a n^2}{V^2}$ → correct, this is the attraction correction term.
• C: $-\dfrac{a n^2}{V^2}$ → not the term as used in the van der Waals equation.
• D: $-nb$ → not correct.

Final answer

The term correcting for attractive forces is $\boxed{\frac{a n^2}{V^2}}$ So the correct option is B.