PRT(10.5.7)(10.5.7)nV=PRT\dfrac{n}{V} = \dfrac{P}{RT} (n/V)(n/V)(n/V) is in both equations. nV=ρM=PRTnV=ρM=PRT \begin{align*} \dfrac{n}{V} &= \dfrac{\rho}{M} \\[4pt] &= \dfrac{P}{RT} \end{align*} Now combine them please. ρM