1. The problem statement, all variables and given/known data I would like to know how to calculate how much moles of gas I have in the following in a cylinder with a certain volume and pressure. The gas in the cylinder is a mixture of air, with added oxygen and helium, the mixture is 18% Oxygen, 36,6% Nitrogen, 45% Helium and 0,4% Argon. The cylinder is 24 liters big, the pressure in the cylinder is 200bar. I can calculate the a and b values for the gas mixture. P = 200 bar V = 24 L T = 293,15 K R = 0,083145 L bar K-1 mol -1 a = 0,8746 L2bar mol-2 b = 0,0388 L mol-1 2. Relevant equations PV = nRT ( ideal gas law) P = nRT (V-nb)-1 - n2a V-2 3. The attempt at a solution According to the ideal gas law this should be 196,93 moles, but that is not right, since if I use the Vanderwaals equation I end up at a pressure of 234,6 bar to accomodate 196,93 moles of this gasmixture. Using the Vanderwaals equation I can't calculate the exact number of moles, this is where is end up: 200 = 24,37n (24 - 0,0388n)-1 - 0,00152n2 So please enlighten me, how can I proceed to calculate the value for n?