[orgo814]

I need to find an expression for the ΔP at constant volume of the van der waals equation. So, since P = RT/Vm - b MINUS a/Vm^2, i have to change P to ΔP. The book gives me the answer ΔP = RΔT/Vm-B. I'm confused about how we got there. I originally got ΔP = RΔT/ΔVm - B MINUS a/ΔVm^2. I know that since volume is constant, the ΔV = 0 and I'm guessing that's how they negated the last term of the equation (a/Vm^2) but how or why does the ΔVm of the first term change back to Vm. If we're assuming that the delta V is equal to 0 which gets rid of the last term of the equation, how can we not assume that it is 0 - b instead of Vm - B on the first term of the equation.