[nawinince]

Which of the following 1) to 4) is correct?

1) The pH of the solution that results when 10ml of 10^-5 mol/l HCl is diluted to 10l with distilled water is 8.

2) The pH of the solution that results when 10ml of 10^-3 mol/l NaOH is diluted to 1l with distilled water is 9.

3) The pH of the solution that results when 10ml of 10^-2 mol/l CH3COOH is diluted to 1l with distilled water is 4.

4) The pH of the solution that results when 10ml of 10^-3 mol/l H2SO4 is diluted to 1l with distilled water is 5.

I can solve 1) and 3) but I have problems with 2) and 4)

2) from CV/1000=CV /1000

(10^-3)*10 / 1000 = C*1000/1000 ---> C=10^-5
then pOH=5 , pH=9

4) from CV/1000=CV /1000

(10^-3)*10 / 1000 = C*1000/1000 ---> C=10^-5
then pH = 5

but the answer is 2). What did I miss in 4)?

[hypervalent_iodine]

When H₂SO₄ dissociates in water, what is the conjugate base? Do you think that conjugate base could undergo further dissociation, and if so, how might that change the pH?

[nawinince]

When H₂SO₄ dissociates in water, what is the conjugate base? Do you think that conjugate base could undergo further dissociation, and if so, how might that change the pH?

Its conjugate will be HSO4 and it will dissociate again and give more H+ so pH will decrease. Right?

[hypervalent_iodine]

When H₂SO₄ dissociates in water, what is the conjugate base? Do you think that conjugate base could undergo further dissociation, and if so, how might that change the pH?

Its conjugate will be HSO4 and it will dissociate again and give more H+ so pH will decrease. Right?

Correct. So when it dissociates, will the concentration of H⁺ be equal to the concentration of H₂SO₄ or will it be greater?

[nawinince]

Correct. So when it dissociates, will the concentration of H⁺ be equal to the concentration of H₂SO₄ or will it be greater?

Greater. So pH will be less than 5?

[hypervalent_iodine]

Correct. So when it dissociates, will the concentration of H⁺ be equal to the concentration of H₂SO₄ or will it be greater?

Greater. So pH will be less than 5?

Yep! Which would therefore make 4) incorrect, and leave you with 2. 

[nawinince]

Correct. So when it dissociates, will the concentration of H⁺ be equal to the concentration of H₂SO₄ or will it be greater?

Greater. So pH will be less than 5?

Yep! Which would therefore make 4) incorrect, and leave you with 2. 

Many thanks!!!

But could you tell me how to calculate pH of 4) ?

[Borek]

Exact, or approximate?

Approximate can be done assuming all HSO₄^- dissociates.

Exact needs taking into account fact HSO₄^- is a relatively weak acid (pK_a2 = 2) and it won't dissociate completely.

For 10^-3 M solution the difference is not large though, just few hundredths of a pH unit.