A vacuum system is measured to have a pressure of 0900 mTorr

A vacuum system is measured to have a pressure of 0.900 mTorr at 22.0 deg. C. How many atoms are in a 1 cc volume of this vacuum at this temperature and pressure?

Solution

Given

Pressure

P=0.9 mTorr

1 torr =133.32 pascals

P=0.9*10^-3*133.32 =0.12 pascals

Temperature

T=22+273.2 =295.2 K

Volume

V=1 cm³ =1*10^-6 m³

From Ideal gas law

PV=nKT

=>n=PV/KT =0.12*1*10^-6/(1.38*10^-23)*295.2

n=2.945*10¹³ atoms