As far as whether the pH will increase, decrease, or stay the same, it simply depends on the nature of the added substance. If you are adding a base, the pH will increase. If you are adding an acid, the pH will decrease. If you are adding a neutral substance, the pH will not change. It does not matter whether you are adding a strong or weak acid or base. Adding a strong acid or base will just change the pH more.
1) If you recognize that NaNO2 is a weak base, then we know that the pH will increase when it is added to the solution. Break it down into its two ions, Na+ and NO2�?/SUP>, the nitrite ion. Na+ is a Group 1A metal ion, so it is neutral. Nitrite ion is the conjugate base of the weak acid nitrous acid, HNO2. Therefore, nitrite ion is a weak base. The conjugate base of a typical weak acid will be a weak base, and the conjugate acid of a typical weak base will be a weak acid.
Adding the weak base nitrite ion will cause the pH to increase. This can also be explained from Le Chatelier's principle. Look at the hydrolysis reaction of the nitrite ion at equilibrium:
NO2�?/SUP> (aq) + H2O (l) 
HNO2 (aq) + OH�?/SUP> (aq)
According to Le Chatelier's principle, if we add more nitrite ion, the reaction will no longer be at equilibrium and will go in the forward direction in order to "counter" the effect of adding extra nitrite ion. If the reaction goes in the forward direction, more hydroxide ion will be formed, increasing the pH. The reaction will go in the forward direction until it is at equilibrium again.
The nitrite ion is "in common" with an ion in the equilibrium. If we added, say, NaCl, there would be no effect because Na+ and Cl�?/SUP> ions are not present in the net ionic hydrolysis reaction. These ion are not in common with any ion in the reaction, so they will have no effect on the reaction or the pH.
2) Since you are adding an acid, the pH will decrease, that is right. Start with the hydrolysis reaction of NaC2H3O2 at equilibrium:
C2H3O2�?/SUP> (aq) + H2O (l) HC2H3O2 (aq) + OH�?/SUP> (aq)
Here, neither H+ or Cl�?/SUP> ion is in common with any ion in the reaction, but the H+ ion reacts with substances in the reaction, which will disturb the equilibrium. First, consider the effect of the added H+ reacting with the acetate ion on the left side:
H+ (aq) + C2H3O2�?/SUP> (aq) ––�?gt; HC2H3O2 (aq)
This extra HC2H3O2 that is formed will drive the hydrolysis reaction in the reverse direction according to Le Chatelier's principle, since HC2H3O2 is on the right side. This results in a decrease in OH�?/SUP> and a decrease in the pH.
Second, you can consider what happens if the added H+ reacts with OH�?/SUP> ion in the hydrolysis equilibrium:
H+ (aq) + OH�?/SUP> (aq) –––�?gt; H2O (l)
The result here is to remove OH�?/SUP> ion. The reaction will go in the forward direction according to Le Chatelier to form more OH�?/SUP>, but more HC2H3O2 will be formed also. The OH�?/SUP> concentration will not be able to get back to its original concentration at equilibrium since the HC2H3O2 concentration is now higher. If the OH�?/SUP> concentration is not as high as originally, neither will the pH.
By the way, since water is the solvent, the effect of forming more water is negligible.
More confused yet? 
Steve
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