Equilibrium Lab Results - titration between weak acid, strong base?

Question

Basically, we had a lab where we preformed titration between a weak acid and a strong base. After the solution went past neutral (> pH 7), the pH level of the solution rose sharply as more base was added, until the solution became highly basic. BEFORE the solution neutralized, the change in pH was VERY gradual.

My question is: Why did the pH of the solution raise so sharply after the solution neutralized?


Any help would be appreciated, I've been trying to interpret the results for a while now with no luck.

Answer 1

You started with a solution of weak acid at a given concentration. Adding a little bit of basic solution will not change the pH much because the acid will react with all the base. since you "lost" a little bit of acis, your pH will raise a little. This is because you end up having a buffer solution (mixture of acid and its base), which are not sensitive to moderate addition of strong base.

When you get close to the equilibrium, you get to a point where the little amount of base you add neutralizes a lot of acid (in percentage of acid remaining). When you neutralize all the acid, you find yourself at pH=7 but then, a few drops of base are enough to bring your solution to a basic pH, because your base is very concentrated.

That explains why you get a graph that looks like a step.

PS: Answer #1 is way off, because HF is a strong acid. A weak acid is acetic acid (found in vinegar). NaOH is a strong base.

Answer 2

It has to do with the buffering ability of the Weak Acid and the crucial point. MY guess is that you titrated a weak acid, like HF, with a strong base, like NaOH. The HF is able to nuetralize some of the OH- ions, making the rise in pH a gradual one initially. Eventually, the critical point is reached, and the OH- ions overcome the HF's Buffering ability (That is, the solution runs out of H+ ions to bind with the OH- ions) to nuetralize them. After this, the OH- causes a sharp increase in pH.

Answer 3

You need to look up "buffer solutions". Before the end-point, that's what you had. After the end-point, you didn't, which is why the pH rose so fast.

Answer 4

I concurr with Nick W. The answer is 42. But Dont Panic!