DP Chemistry (last assessment 2024)
Question 19M.3.hl.TZ2.10a(i)
| Date | May 2019 | Marks available | [Maximum mark: 1] | Reference code | 19M.3.hl.TZ2.10a(i) |
| Level | hl | Paper | 3 | Time zone | TZ2 |
| Command term | Outline | Question number | a(i) | Adapted from | N/A |
a(i).
[Maximum mark: 1]
19M.3.hl.TZ2.10a(i)
Amino acids contain both acidic and basic functional groups.
An aqueous solution of glutamine contains 0.600 mol dm−3 of the zwitterion and 0.300 mol dm−3 of the anionic form.
pKa1 = 2.2 and pKa2 = 9.1
(a(i))
Outline which pKa value should be used when calculating the pH of the solution, giving your reason.
[1]
Markscheme
pKa2 AND pH of solution > pH of isoelectric point «as anion present»
OR
pKa2 AND zwitterion has lost H+ to become anion «so in basic solution»
OR
pKa2 AND «only» anion «and zwitterion» present [✔]
Examiners report
Several candidates stated correctly that pKa2 should be used with a reason whereas others wrote pKa1, which was incorrect.
