A mixture of three noble gases has a total pressure of 1.25 atm. The individual pressures exerted by neon and argon are 0.68 atm and 0.35 atm, respectively. Wha t is the partial pressure of the third gas, helium?

This problem can be solved by Dalton's law of partial pressures which states that in a mixture of non-reacting gases, the total pressure exerted is equal to the sum of the partial pressures of the individual gases.

Ptotal = P₁ + P₂ + P₃ + ... + Pn

Herein, Ptotal = P (Ne) + P (Ar) + P (He)

Ptotal = 1.25 atm, P (Ne) = 0.68 atm and P (Ar) = 0.35

So, P (He) = Ptotal - P (Ne) - P (Ar) = 1.25 - 0.68 - 0.35 = 0.22 atm