Question

A glass tube of volume 112 ml. containing gas is partially evacuated till the pressure in it drops to $3.8\times {10}^{-1}$ torr at ${25}^{o}C$. The number of molecules of the gas remaining in the tube is:

• A. $3\times {10}^{17}$
• B. $1.5\times {10}^{18}$
• C. $4.5\times {10}^{18}$
• D. $6\times {10}^{18}$

Answer 1

The correct option is B $1.5\times {10}^{18}$

Volume= $112ml=0.112litre$

Pressure= $3.8\times {10}^{-1}torr=0.0005atm$

$T=0^{o}C=273K$

$PV=nRT$

$\Rightarrow n=\frac{0.0005\times 0.112}{0.0821\times 273}=2.498\times {10}^{-6}$ moles

The number of molecules of the gas remaining in the tube= $2.4018\times {10}^{-6}\times 6.023\times {10}^{23}=1.5\times {10}^{18}$