# Chemistry A level pH help

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#1
A solution of glycolic acid of concentration 0.1 mol dm–3 has a pH of 2.4
What is the approximate pH of the resulting solution after it has been diluted by a factor of 100?
A 1.4 B 2.4 C 3.4 D 4.4

Would really appreciate it if someone could help me with this. The mark scheme says its C but I thought it was D. I originally did 3.981x10^-3 divided by 100 to get 3.981x10^-5. I then -log this answer to get a pH of 4.4. Not sure why this is wrong?
0
4 months ago
#2
(Original post by Thomas.5)
A solution of glycolic acid of concentration 0.1 mol dm–3 has a pH of 2.4
What is the approximate pH of the resulting solution after it has been diluted by a factor of 100?
A 1.4 B 2.4 C 3.4 D 4.4

Would really appreciate it if someone could help me with this. The mark scheme says its C but I thought it was D. I originally did 3.981x10^-3 divided by 100 to get 3.981x10^-5. I then -log this answer to get a pH of 4.4. Not sure why this is wrong?
The reason for this unexpected result lies in the fact that Glycolic acid is a weak acid. Although the Ka is constant, the proportion of acid which dissociates increases as the solution becomes more dilute. See working out below:

This would not be strude for a strong acid- all of it dissociates, so if you diluted it 100 x the pH would go up 2 units, diluted it 1000 times it would go up 3 units etc.

This is a very sneaky question- can't imagine it being on a real A-level paper.
1
#3
Thanks so much for taking the time to go through it step by step. That clears things up!
(Original post by Davies Chemistry)
The reason for this unexpected result lies in the fact that Glycolic acid is a weak acid. Although the Ka is constant, the proportion of acid which dissociates increases as the solution becomes more dilute. See working out below:

This would not be strude for a strong acid- all of it dissociates, so if you diluted it 100 x the pH would go up 2 units, diluted it 1000 times it would go up 3 units etc.

This is a very sneaky question- can't imagine it being on a real A-level paper.
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