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# A flask containing 12g of a gas of relative molecular mass 120 at a pressure of 100 atm , was evacuated by means of a pump untill the pressure was 0.01 atm at the same T. Which of the following is the best estimate of the number of molecules left in the flask. $(N_0 = 6\times10^{23} mol^{-1}$)

$(a)\;6\times10^{19}\qquad(b)\;6\times10^{18}\qquad(c)\;6\times10^{17}\qquad(d)\;6\times10^{13}$

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## 1 Answer

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At constant V and T
$\large\frac{P_1}{n_1} = \large\frac{P_2}{n_2}$
$\large\frac{100}{\large\frac{12}{120}} = \large\frac{0.01}{n_2}$
$(n_1 = \large\frac{12}{120})$
$n_2 = \large\frac{0.01}{1000}$
$\;\;\;\;=1\times10^{-5}$
No. of molecules left = $6.02\times10^{23}\times10^{-5}$
$\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;=6.02\times10^{18}$
Hence answer is (b)
answered Mar 5, 2014

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