How to Calculate the pH of a Buffer Solution After Adding HCl?

[nobb]

Hi.
I was wondering if someone could please explain how to do this question. Find the pH of 1 L buffer (1 mol/L CH3COOH, 1 mol/L CH3COO-) when 0.1 mL 1 mol/L HCl is added. Help would be appreciated. Thanks in advance.

 

[Renge Ishyo]

You are dealing with a weak acid, so you have to use the methods to solve for the H^+ concentration at equilibrium. I think the missing value from the question statement is that you need to use the K_a for acetic acid (which, if my memory doesn't decieve me, is something like 1.8x10^-5).

The HCl essentially disassociates completely; so when you are setting up your equilibrium table, you can treat the intial concentration of HCl as the initial conentration of "H^+" ions. When you are done, you can plug in your new values of CHOO- and CH3COOH into the relationship below and solve for the H^+ concentration (which you can then convert to pH):

1.8x10^-5 = [H^+][CHOO^-]/[CH3COOH]

Hope this all seems familiar to you ^^

 

[GCT]

that is the HCl will react with the conjugate base, which is consumed, and in turn produces more conjugate acid. Find the new concentrations, plug in, and solve for the hydronium ion concentration.

 

[Borek]

http://www.chembuddy.com/?left=pH-calculation&right=pH-buffers-henderson-hasselbalch

Assume all added HCl was consumed in stoichiometric reaction with CH₃COO^-.