Partial Pressure and Partial Molar Volume When we define partial pressure and partial molar volume of a gas in a mixture, suppose $P$ is partial pressure of a gas in a mixture and $V$ is partial molar volume 
then why the following equation doesn't hold
$PV=nRT$, (where $n$ is number of moles of gas) ?
It doesn't work because partial molar volume of a species in a gas mixture is defined as the partial derivative of the total volume of the system with respect to the number of moles of that species at constant total pressure and temperature of the system.