Say I have got a super cooled cylinder of half hydrogen and half helium. This cylinder has a pressure of 100,000 Pascal's. At this pressure, hydrogen boils at 20 Kelvin and helium at 4.21 Kelvin. I hope to separate helium by cooling gas down to 4.21 Kelvin but I know that even at the temperature, some hydrogen will still evaporate. I hope you can introduce me to the formulas used to find the amount of hydrogen that will evaporate along with helium, therefore allowing me to find the purity of the hydrogen I will have obtained. Please do not be discouraged with having to teach me basic physics if it's called for.