Question

A flask of volume $1$ litre contains vapour of $C{H}_{3}OH$ at a pressure of $1atm$ and ${25}^{o}C$. The flask was then evacuated till the final pressure dropped to ${10}^{-4}mm$. Find the number of molecules of methyl alcohol left in the flask.

Answer 1

$V=1lit,P=1atm$ & $T={25}^{o}C=298K$

$Pf={10}^{-4}mm=\frac{{10}^{-4}}{760}atm=1.316\times {10}^{-7}atm$

$PV=nRT$

$n=\frac{PV}{RT}=\frac{1.316\times {10}^{-7}\times 1}{0.0821\times 298}=5.38\times {10}^{-9}$

$\therefore$ No. of molecules $=n.N_A=5.38\times {10}^{-9}\times 6.023\times {10}^{23}=3.24\times {10}^{15}$