Question

A gas mixture with total pressure of 300 torr consists of equal masses of Ne(atimc weight 20) and Ar (atomic weight 40). What is the partial pressure of Ar in torr?

Answer 1

the total pressure is equal to the sum of the partial pressures of both gasses.
Ptotal=PNe+PAr
Ideally the ratio of partial pressures is the same as the ratio of molecules. This is done by using the mole fraction.
So the partial pressure of the Ar gas component in an ideal gas can be obtained using this expression:
PAr=mole fraction*Ptotal.
To find the mole fration we know that both have equal masses, but since Ar weighs twice as much as Ne that means that there are half as many molecules of Ar as there are for Ne. So that means that if there is one moel of Ar then there are 2 moles of Ne. The total moles is 3 so the mole fraction is 1/3.
PAr=1/3 * 300 torr
PAr=100 torr.
Which is your final answer.