Chemistry - I'm pretty sure this doesn't follow Le Chatelier's Principle

Solution 1:

Le Chatelier's Principle does not directly say what happens to concentration ratios. Nor does it directly compare conditions after a disturbance with those before. It says what happens only after a disturbance has been applied.

Here, the disturbance is adding one mole of A per liter. When equilibrium is re-established you have only an additional one-half mole of A per liter, the rest having reacted to form B. The initial addition of A has been partially offset, which is exactly what Le Chatelier's Principle says.

Solution 2:

You are simply slightly misapplying Le Chatelier. The principle is used to predict the reaction of a system to changes in the external conditions, i.e. temperature or pressure, not to you wilfully changing the internal composition of the system.

The general idea (system minimises the change) is of course right, but at constant conditions, the law of mass action allows a direct quantitative predicition, so why bother with Le Chatelier?

Anyway when the system reaches equillibrium after your addition of compound A, the relative change in the system is already zero again, so, following Le Chatelier, the change in the equillibrium constant would also be nil. No contradiction at all.