[indianfighter]

Hello!

This is my first post on this forum. I thought it may be worth the try. I am doing some exercises about pH and buffer solutions for the oncoming exam.

This is the problem I am a bit confused about:

a) What type of system is made by mixing 250 mL of acetic acid solution of 0,25M concentration with 300 mL of potassium hydroxide solution of 0,1M concentration? What is the pH of this mixture? (Ka(acetic acid) = 1,86*10^-5)

I think I have solved this one correctly by using the Henderson Hasselbach equation. I got pH = 4,69.

b) How does the pH of the above mixture change if 450 mL of hydrochloric acid solution of 0,03M concentration is added?

Ok, the molar amount of added HCl is 0,0135 mol. From task a) I have 0,0325 mol of acetic acid and 0,030 mol of CH3COOK after the equilibrium is reached. I have tried adding the molar amount of acetic acid already in the mixture to the molar amount of HCl added (0,0325 mol + 0,0135 mol = 0,046 mol) and, then used this as the acid concentration in the Henderson Hasselbach equation. I got 4,54, but I think it's not correct. How come so little change in pH after adding so much HCl?

I am very thankful for any help and explanations.

Sandip

[Borek]

Strong acid protonates weak acid conjugate base.