Question

How many molecules of N2 remain in an ultrahigh vacuum system of 128 mL volume when the pressure is reduced to 5*10^-10 mmHg at 25°C?

Answer 1

volume = 128 mL = 0.128 L

pressure = 5 x 10^-10 mmHg = 6.58 x 10^-13 atm

temperature = 25 oC = 298 K

P V = n R T

6.58 x 10^-13 x 0.128 = n x 0.0821 x 298

n = 3.44 x 10^-15

moles of N2 = 3.44 x 10^-15

number of molecules = 3.44 x 10^-15 x 6.023 x 10^23

                                  = 2.07 x 10^9 molecules