Question

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

Answer 1

First of all, as you seen the gases are noble which means that will not react with each other and in this case each gas create individual pressure.  

P$_{T}$= total pressure  

P$_{Ne}$ = pressure of neon  

P$_{Ar}$ = pressure of argon  

P$_{He}$ = pressure of helium {which is required}

P$_{T}$ = P$_{Ne}$ + P$_{Ar}$ + P$_{He}$    

1.25 = 0.68 + 0.35 +  P$_{He}$

P$_{He}$   = 1.25  - [0.68 + 0.35] = 0.22 atm