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.


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