Le Chatelier's Principle

Question

Some solid NH4Cl is added to a water solution of NH3. Which of the following statements is correct?

1) [OH-] of the solution remains the same

2) [NH3] of the solution decreases

3) [OH-] of the solution increases

4) [H3O ] of the solution increases

5) [H3O ] of the solution remains the same

can you explain me why # 4 would be correct and explain why each of the others are not. I think this has to do something with Le Chatelier's Principles that im not really understanding it

Answer 1

This principle states that when the rates of forward and reverse reactions will change to relieve stress. So if you are adding NH4Cl to the solution of NH3, you are increasing the equilibrium which causes a shift to balance it out. Therefore, the H3O remains the same because it balances itself.  

Answer 2

NH3 + H2O <===> NH41+ + OH-

Adding NH4Cl is, in effect, adding NH41+ ions to the system. The equilibrium will shift to the left as the system tries to use up some of the added NH41+ ions (LeChatelier's principle). When the equilibrium shifts to the left, more NH3 and water form, while the amount of OH1- will decrease. I don't really like any of the choices given. But, if the OH1- concentration decreases, then the H3O+ concentration increases.