Question

A 1.00 liter flask filled with a gas at STP is attached to a vacuum pump and evacuated until the pressure is mm Hg. Assuming no temperature change, how many molecules remain in the flask?

Answer 1

$pV=nRT$

Here it is said that there is no temperature change hence T will remain as a constant and there is flask whose volume is 1 litres so it will remain to be 1 litres hence V will be also treated as constant

Finally we get a equation

$\frac{p}{n}=constant$

In the Initial condition gas is at STP so lets calculate the initial mole from this

$n=\frac{pV}{RT}=\frac{1\times1}{0.082\times273}=0.045$

Let's apply the equation $\frac{p_1}{n_1}=\frac{p_2}{n_2}$

$\frac{760}{0.045}=\frac{p_2}{n_2}$

$n_2=\frac{p_2\times0.045}{760}moles$

number of molecules will be equal to n₂N_A