if 0.4M NaOH is titrated with 0.4M HF, how do we calcualte the ph at equivalence. The book assumes each is 1L, but why do we use 1L

* chemistry - Dr.Jim, Thursday, November 11, 2010 at 5:31am

HF is a weak acid, so you need the dissociation constant. The pH at the equivalence point is found by calculating the pH of a 0.4M NaF solution (a hydrolysis problem). If you erroneously assume that HF is a strong acid, then the answer is simple. The pH at the equivalence point is 7.

how can a weak acid and strong base titration has ph=7 at equivalence. It's only when they are both strong acid and base

I had trouble knowing where your question ended and Dr Jim's response ended. I think you asked one question not answered previously; why 1 L was chosen?

Why 1L? It's a convenient number to chose. You could have chosen any other number and the pH would be the same.

If you asked another question buried in there somewhere, please post again at the top of the page and someone will pick it up.

To calculate the pH at the equivalence point of a titration between a weak acid (HF) and a strong base (NaOH), you need to consider the reaction that takes place when they react.

In this titration, the weak acid HF reacts with the strong base NaOH to form the salt NaF and water:

HF + NaOH -> NaF + H2O

Initially, when the titration begins, the solution contains 0.4M NaOH. As you mentioned, the book assumes each solution has a volume of 1L, so we will work with that assumption.

At the equivalence point, the moles of acid and base that react are stoichiometrically equal. Since the concentration of both solutions is 0.4M and the volume is 1L, the number of moles of HF and NaOH will be the same.

Now, let's consider the dissociation of the salt NaF (formed at the equivalence point) when it dissolves in water.

NaF -> Na+ + F-

The F- ion can undergo hydrolysis in water, resulting in the formation of OH- ions. This process creates a basic solution.

Since F- is the conjugate base of a weak acid (HF), it will react with water to produce OH- ions:

F- + H2O -> HF + OH-

This hydrolysis reaction will produce OH- ions and increase the pH of the solution.

Therefore, at the equivalence point, the solution will have a higher pH than 7. The exact pH value will depend on the concentration of the NaF solution and the extent of the hydrolysis.

So, the claim made in the book that the pH at the equivalence point is 7 is incorrect for the titration between a weak acid and a strong base.