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The whole point of the graph is that $pV/(RT)$ is no longer equal to $n$ for a real gas. $n$ isn't changing (as you point out, this is physically impossible), but the ratio $pV/(RT)$ is changing to some value that is not the same as $n$. Once you're freed from the ideal gas equation, there's no need for $n$ to "follow" the changes in pressure / ...


1

Only for ideal gases the equation $pV = nRT$ holds good. Therefore $pV/RT = n$ is valid only for ideal gases. And in the graph, $n$ is constant for ideal gas. For the real gases $\ce{CH4}$, $\ce{N2}$, $\ce{H2}$, $\ce{CO2}$, while the amount of substance $n$ is kept constant, the ratio $pV/RT$ is not a constant anymore ($pV/RT \ne n$). Instead, for real ...


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