Chemistry A level pH help

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.