The Attempt at a Solution
I am confused on getting the molar mass of the mixture, unless the answer is wrong.
For nN2 I done 0.79*(3*10-3) which gave me 2.37*10-3kmol
For nO2 I done the same but with 0.21 to get 6.3*10-4kmol
The ntotal of the mixture is 3.5*10-3kmol
For each pressure I used p=nRT/V with R=8314 to get 174.594kPa for the total pressure, 31.427kPa for the partial pressure of oxygen, and 118.225kPa for nitrogen.
But when working out the partial pressure of CO2 I got 24.942kPa. In the answers it states it is actually meant to be 29.94kPa but unless this is a mistake, I am not sure why it would be this.
Lastly for the Molar mass, I done (mO2/ntotal)+(mN2/ntotal)+(mCO2/ntotal) to find the total molar mass, and that gave me 14.455kg/kmol, when it should apparently be 31kg/kmol.
I done 1/3*(0.5*10-3)(16) to find the mass of Carbon and used 2/3 for the mass of Oxygen in the CO2.