[ghostanime2001]

Why is it that, when you add a strong acid to a buffer solution of sodium acetate and acetic acid the # of moles of added strong acid is completely consumed by the acetate ions ? Assuming the # of moles of strong acid is much less than acetate ions.

[fledarmus]

1) What species do you have in solution in your buffer?

2) What mathematical relationships do you know that relate to the concentrations of those species?

3) What happens to your strong acid when you add it to the solution?

4) How does this affect the mathematical relationships described in 2)?

[ghostanime2001]

Lets suppose there is a solution of acetic acid at equilibrium and then you add HCl. I know that the acetate ions will consume the added hydronium ions and produce more acetic acid. But why is it that whatever number of moles of hydronium ions from HCl completely reacts with whatever moles needed with acetate ions ?

[fledarmus]

There is an equilibrium that describes the relative concentrations of H⁺, OAc^-, and HOAc. Sodium acetate will completely dissociate in water, and hydrogen chloride will completely dissociate in water.

So as long as you have considerably more acetate ion from your sodium acetate in the water, as you add hydrogen chloride, the acetate ions will react with the hydrogen ions to reestablish the equilibrium.

[Borek]

But why is it that whatever number of moles of hydronium ions from HCl completely reacts with whatever moles needed with acetate ions ?

In general it doesn't. But that's a quite good approximation.

[ghostanime2001]

Then what is your way of doing it ?

[Borek]

In most cases I assume that's correct - that is, weak acid gets quantitatively protonated. I know it may not work for strong acids (say pKa below 2, or 3) and in the case of diluted solutions (say below 10^-4M) - in such cases to be sure (and if it matters) I check if the results are reasonable. In most cases I do it using ChemBuddy calculators (after all, I know how they work), but there are methods that don't require any software. Sorry, no time now to prepare an example - but in general you just have to check if the calculated values fit mass balances, charge balances and all equilibrium constants. If the assumption is wrong, results won't meet these criteria.