# Why does final pH value of buffer solution after addition of acid not depend on amount added?

I'm brushing up and the example in my book is against my intuition.

A buffer solution contains 1.0 mol/L acetic acid and 1.0 mol/L sodium acetate. The solution is at pH = pKa = 4.75. What's the pH after addition of 0.01 mol/L HCl?

They don't write how much of the acid is added.

They go on stating that addition of x = 0.01 mol/L H+ results in pH = 4.75 - log[(1.01 mol/L) / (0.99 mol/L)] = 4.733.

That's all fine from the formalism. What's against my intuition is:

Why is it not required to state how much of the acid was added? Given the buffer solution, is the outcome of adding acid the same independent if I add a Liter of if I add a mL?

• maybe they meant "addition of 0.01 mol" and the "per liter" is a mistake? at least that would make sense. – pH13 - Yet another Philipp Mar 2 '16 at 22:51
• The concept of a buffer is that a small addition of acid or base won't change the pH significantly. The pH must change at least a small amount. – MaxW Mar 3 '16 at 2:11
• @MaxW I don't understand your comment. My question is not about what a buffer does. It's about why the amount of acid is not required in the question to calculate the change in pH. Will the pH change be the same no matter how much acid is added? – TMOTTM Mar 3 '16 at 19:20
• When more acid is added, then the pH must fall - at least a little. But a drop of conc. HCl doesn't change pH of 100 ml of pH 7.0 buffer much, but it changes pH of 100 ml of pure water a lot. – MaxW Mar 3 '16 at 19:33