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. $(N_0=6\times {10}^{23}mo{l}^{-1})$?

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

Answer 1

Answer: (B)

$6\times {10}^{18}$

$P_1{V}_1=n_{1}R{T}_1$
$P_2{V}_2=n_{2}R{T}_2$
$\frac{n_2}{n_1}=\frac{P_2}{P_1}$
$n_2=0.1\times \frac{0.01}{100}={10}^{-5}mol=6\times {10}^{18}molecules$