We know that ideal gas equation is

PV=nRT

Where V= volume of gas

R=gas constant

T=temperature

n=number of moles of gas

P=pressure of gas.

Gas constant R=kN_A

Where k=Boltzmann constant

N_A=Avogadro number

So, we can ideal gas equation as

PV=nkN_AT

Now, we know that

n=number of molecules of gas ……(ii)

Therefore, from equation (i) and (ii)

Number of molecules of gas=