[qwerty12321]

Hi Everyone,

I was just wondering if you know of any factors that would affect the dissociation of a strong acid. I know that weak acids are affected by factors such as the common ion effect and temperature, but would this apply to strong acids as well?

I only ask because we added sulfuric acid to a solution and then measured the pH. The [H+] indicated by the pH meter suggests that there are less H+ ions than would be from the concentration of sulfuric acid added. Any suggestions for a cause of this would be greatly appreciated!

Many thanks.

[Borek]

Second dissociation step for sulfuric acid is not as strong as you think.

Assumption that strong acids are fully dissociated is only an approximation. Usually a good one, but approximation.

[qwerty12321]

Sorry, i'm kind of confused. Basically what happened is we added approximately 10mL 1.8M sulfuric acid, and the pH was 0.71. This suggests that the [H+] concentration is at least 1.95M. Where did the additional H+ ions go?

[SirRoderick]

To what did you add it?

[qwerty12321]

The solution contains NaOCl (which dissociates into OCl-, HOCl, H+ and NaOH) and the sulfuric acid.

[rabolisk]

While there might be some HOCl, there won't be a lot. Also, there will be practically no NaOH.

Still I'm a little confused about your question. The pH of the resulting solution (after adding acid) will be dependent on how much NaOCl solution you had in the first place. If I add 10 mL 1.8M sulfuric acid to the pacific ocean, will I see any change in pH? Also, since OCl- is a base, it would be important to know its concentration. We can't answer your question without more information. Finally, pH of 0.71 corresponds to roughly 0.195M, not 1.95M.

[qwerty12321]

Okay, sorry - here goes:

1. Added 25mL of 10% NaOCl solution (diluted 1:50)
2. Added 10mL of KI (shouldn't affect) and 10mL of approximately 2.0M SUlfuric acid (probably a bit less concentrated, I am checking this).
3. Measured the pH of the solution at different temperatures to determine the extent of dissociation of HOCl (from NaOCl)

However, as I said, the pH of some solutions (colder ones - 18ish degrees C) suggest that the concentration of the H+ ions is less than what should be formed by the dissociation of sulfuric acid (strong acid).

For example;
pH=0.71
therefore, H+ should be at lesat 0.195 (with 0.0088 moles, due to total 45mL solution)
To find the number of moles H+ from HOCl, we substracted this value (total H+) from the number expected from the disociation of sulfuric acid (approx. 0.01878 moles - 10mL of 1.278M).

However, ended up with -0.010005699 moles!

Any ideas?

[enahs]

You are taking 10mL of, at most 2M solution of H2SO4.
So you have added at most 0.01 * 2 = 0.02 mols of H2SO4.

If you assume that it completely dissociates (which is false, see Boreks previous post), you still have added at most 0.04 mols of H+. That is mols, not molarity.

I have no clue what your final volume is. First you say add 25mL and it is a 1:50 dilution, and then you say later total 45mL solution...

Anyway, that is irrelevant, other then I am trying to figure out what you are doing, you are still not very clear.


But you said "suggest that the concentration of the H+ ions is less than what should be formed by the dissociation of sulfuric acid (strong acid). " have you read Broeks post? He specifically states that the second dissociation is not fully dissociated.
Also note, that pH meters are only accurate at the temperature they are calibrated at (and basic pH meters have a limited temperature range they are accurate at).  And remember pH is log scale, so just reading it wrong by 0.01 is actually a substantial change in [H+].