# Homework Help: Calculate the pH of 20.00 mL of 0.20 M KHP

1. Nov 5, 2009

### w3390

1. The problem statement, all variables and given/known data

Calculate the pH of 20.00 mL of 0.20 M KHP.

Given: pKa1(H2P)= 2.950 pKa2(H2P)= 5.408

2. Relevant equations

k = [H][P]/[HP]

3. The attempt at a solution

I used this equation and got:

3.91e-6 = [x][x]/[.2]

x^2 = 7.82e-7

x = 8.8e-4 = [H]

pH = 3.05

However, my homework is telling me this is incorrect. Is it just a rounding issue or is it something else? Any help is much appreciated.

2. Nov 5, 2009

3. Nov 5, 2009

### symbolipoint

Re: titrations

Either your pK1 value is wrong OR your K1 value is wrong. Fix the wrong one and you may be able to improve your result. I base my comment on the idea that the first dissociation is far more important than the second dissociation in the solution just containing the dissolved KHP. I may be incorrect in this judgement (while still relearning).

Last edited: Nov 5, 2009
4. Nov 6, 2009

### Staff: Mentor

Re: titrations

This is Ka2 and pKa2. w330 tries to base calculations only on the second dissociation step, ignoring hydrolysis. As explained on the linked page this is incorrect approach.

--
methods

5. Nov 6, 2009

### symbolipoint

Re: titrations

Borek, the discussion in your hyperlink in the post was interesting, and a little clearer than the discussion in the old analyitical textbook I read yesterday. Something almost like it was also shown in an old General Chemistry textbook. I have not seen much of that type of exercise for a long, long time.

6. Nov 6, 2009

### Staff: Mentor

Re: titrations

In a way I am like an old analytical chemistry textbook :grumpy: