Question

A flask containing $12\, g$ of a gas of relative molecular mass $120$ at a pressure of $100 \, atm$ was evacuated by means of a pump until the pressure was $0.01\, atm$. Which of the following is the best estimate of the number of molecules left in the flask $\left( N _0=6 \times 10^{23} \, mol ^{-1}\right)$ ?

• A. $6 \times 10^{19}$
• B. $6 \times 10^{18}$
• C. $6 \times 10^{17}$
• D. $6 \times 10^{13}$

Answer 1

Answer: (B)

$P_1 V_1=n_1 R T_1$
$P_2 V_2=n_2 R T_2$
$\frac{n_2}{n_1}=\frac{P_2}{P_1}$
$n_2=0.1 \times \frac{0.01}{100}=10^{-5} mol $
$=6 \times 10^{18}$ molecules