I was given the following formula which is used for ideal gasses: $$pV = nRT$$


  • p:pressure
  • V: volume
  • n: number of moles
  • R: gasconstant
  • T: temperature

now when speaking about vaporpressure curves like this one:

enter image description here

You can change the formula like so: $$ pV = NkT = nRT = \frac{m}{M}RT$$


  • V: vapor volume
  • m: mass of the vapor
  • M: molar mass of the particles
  • N: number of particles which are in vapor state
  • n: amount of moles

Based on the previous formula you can calculate the pressure like this: $$\rho(T) = \frac{pM}{RT}$$

Now I would like to know where the volume is in this final formula to calculate the pressure?

Is this formula still applicable: $p = \frac{nRT}{V}$ ? (If not, in which cases is it?) This formula seems much easier to me.

  • $\begingroup$ Please define $\rho$ $\endgroup$
    – pentane
    Commented Jan 22, 2016 at 13:43

1 Answer 1


The answer you are looking for is that \begin{equation} \rho \propto 1/V \end{equation} and \begin{equation} m/\rho = 1/V \end{equation} with this knowledge you should be able to see how your final equation relates to \begin{equation} pV = nRT \end{equation}


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