Help with Acid-Bases

Hello.
I have a question on Acid bases, specifically the one on this link - https://www.thestudentroom.co.uk/showthread.php?t=923227
I do not understand how the moles of weak acid and strong base can be the same if the weak acid dissociates less than the strong base? Then the moles oh H+ shouldn't equal the moles of hydroxide ions right?

Reply 1

TypicalNerd

Original post by mitostudent

That question is simply using Ka = [H^+][CN^-]/[HCN]

Where [H^+] ≈ [CN^-] because even though it’s a weak acid, it’s waaaay more acidic than water (HCN pKa = 9.40 vs H2O pKa = 14) and so pretty much all the hydrogen ions come from the ionisation of HCN, which reduces the ionisation of water. But since there is little dissociation, we can assume that [HCN] = 0.01 mol dm^-3 and there is no need to account for a change in [HCN] due to ionisation.

Thus we have 10^-9.40 = [CN^-]^2 / 0.01

=> [CN^-] = 1.995… x 10^-6 mol dm^-3

≈ 2 x 10^-6 mol dm^-3

What you have described is in no way related to the link. It seems to be a misunderstanding of both buffer solution calculations.