Hi all, i've encountered this question below during my coursework...

In an experiment, x moles of A placed in a flask underwent a reaction to produce B.
A(g) <-> 2B(g)   [reversible reaction]
At equilibrium, y moles of A had reacted and the total pressure in the flask was P atm.

If z moles of inert C is added to the closed flask, how will the value of equilibrium constant and the position of equilibrium be affected?

I've worked my answer out for this and my answer differs from my tutor's regarding the position of equilibrium. I strongly think that the position of equilibrium will shift left as by Le Chatelier's Principle, the closed system will relieve the stress (due to the addition of C molecules, therefore the backward reaction is favoured.

However, my tutor claimed otherwise and said because C is inert, it doesn't affect the system's equilibrium position.

Please advise and convince me that I'm wrong.  

Wrong!
The addition of inert C does not alter the partial pressures of the reactants. No shift in equilibrium. If a shift occured, the K wouldn't be the same value.

If the pressure was altered through compression or expansion, the partial pressures of the reactants would change and a shift would occur.

but according to Chatelier principle a shift will undo an effect on the equilibrium so that the system has the same K after equilibrium is reached again.

But as we have more C the mole fractions of the equilibriumparticipants changes more for B then for A.

I've one part which i don't get it "What counts here are partial presures, not the total system pressure."

Why doesn't the total system pressure matter? Isn't the partial pressure derived from the total system pressure?

So am i right to say now that if an inert gas is added to a close system, the respective partial pressure of the gas A, and B doesn't change?

Then under what circumstances will the partial pressure change?

Because i'd always thought that since P_A=P n_A/n_(A+B) then when i add n_c of inert gas into the system, the partial pressure of the gases will be redistributed such as P_A=P' n_A/n_(A+B+C) giving a different result since the mole ratio and total pressure have changed.

pressure is the collision of molecules to a surface. When there are more molecules in a same volume there are more collision to the surface and thus the pressure rises.