Student Doctor Network Forums > Pre-Medical Forums > MCAT Discussions > Kaplan MCAT Forum > physical sciences question...gasses in a system PDA View Full Version : physical sciences question...gasses in a system 24486905-19-2009, 09:56 AMI'm confused on a particular question in one of the kaplan full lengths. so we are given: CO2(g) + C(s) = 2 CO(g) The question is: If a sample of Helium gas was injected into the system, what would happen to the amount of carbon dioxide in the system? Kaplan's answer is no change. Because Helium is noble gas and therefore unreactive. I thought it would increase CO2 because...the total pressure of the system is increased which prompts the reaction to decrease pressure by proceeding in the direction of lesser gas molecules. Can anyone help me understand why I am wrong? MD2B in 201005-19-2009, 12:15 PMWell I mean, if you look at the equation, you can see that none of the variables, reactants nor products contain helium in the equation. That should prompt you in thinking well what would this new element do. Also this is probably a question in regards to Le Chatelier's Principle, so when you think about all the things that can change the system (pressure, temperature, moles, volume, etc..) where does Helium come into the equation? It appears to not because it is not even in the equilibrium formula to affect the system kind of like with water and diluting an acid and pH, it does not change. Also, as Kaplan says, it is an inert gas. Hope this helps ya. VanBrown05-19-2009, 12:33 PMDoes it give you the Keq of the reaction? 2 CO ---> C (s) + CO2 isn't going to happen readily... try to apply Le Chat only to reactions that are completely in the gas phase when trying to decide what an increase in partial pressure will do. anxiousteen05-19-2009, 01:39 PMin this case, le chatlier's principles applies to concentration. does adding helium to the container change the concentrations of the reactions or products? no 24486905-19-2009, 02:22 PMOk. I think I understand now. So even though adding Helium into the system raises the total pressure of the system...the partial pressures of the gasses do not change because Helium is an inert gas. BUT lets say the volume of the system decreased causing the total pressure to increase...this changes the partial pressures of the gasses which would then prompt a shift to less moles of gas. Am I getting this right? SzeatKaplan05-19-2009, 02:23 PMHey DPMHopeful: This is one of the wonderful exceptions of Le Chatelier's principle. While adding an inert gas at a constant volume will change the total pressure of the system, the partial pressure of the other gases do not change. It is the change in partial pressures that causes the equilibrium to shift; therefore, to get a shift, one must change the volume. 24486905-19-2009, 07:39 PMThanks everyone for your input! Ommu05-22-2009, 10:51 PMGood explanation, thanks.