# Finding partial derivatives for an ideal gas and van der Waals gas

How do I determine $\left({\partial P\over\partial V_\mathrm m}\right)_T$ for an ideal gas and also for a van der Waals gas? Note that $T$ is the only variable which is held constant, and that's my trouble. I've only derived these equations in cases where two variables are held constant. I don't know where to start.

• Roughly speaking, if two variables are held constant, everything is held constant, so I'm not quite sure what do you mean. – Ivan Neretin Sep 29 '15 at 14:23

Use the equation of state, express $P$ as a function of $T$ and $V$, and find the derivative with respect to $V$, assuming $T$ to be constant.