Helium Pycnometry

Question 1

Describe what happens in helium pycnometry.

Answer 1

~ a sample of unknown volume and known mass is put into the sample chamber and the lid is sealed

~ the sample chamber is at pressure Ps

~ the sample chamber is connected to the reference chamber through a valve

~ the reference chamber has a known Vr and is at pressure Pr

Question 2

What is the equation when the valve is closed initially?

Answer 2

~ Ntot = Nsample chamber + Nreference chamber
~ Ntot = Ps(Vs-Vx)/RT + (PrVr)/RT

~ Ntot =moles gas

Question 3

What happens when the valve is opened?

Answer 3

~ the pressure equilibrates

~ Psystem(Vs-Vx+Vr) = Ntot*RT

~ Vx = (PsystVs + PsystVr - psVs - prVr) / (Psyst - Ps)

~ px = Mx/Vx

Question 4

Why is helium a preferred gas?

Answer 4

~ it readily penetrates into small open pores