Question 19M.3.hl.TZ2.10
| Date | May 2019 | Marks available | [Maximum mark: 4] | Reference code | 19M.3.hl.TZ2.10 |
| Level | hl | Paper | 3 | Time zone | TZ2 |
| Command term | Calculate, Describe, Outline | Question number | 10 | Adapted from | N/A |
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
Outline which pKa value should be used when calculating the pH of the solution, giving your reason.
[1]
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 [✔]
Several candidates stated correctly that pKa2 should be used with a reason whereas others wrote pKa1, which was incorrect.
Calculate the pH of the glutamine solution using section 1 of the data booklet.
[1]
«pH = 9.1 + log »
«= 9.1 + (–0.3)» = 8.8 [✔]
Majority of the candidates calculated correctly the pH of the glutamine solution while other candidates managed the ECF mark from part a (i).
Describe what is meant by the genetic code and how it relates to protein synthesis.
[2]
sequence of bases in DNA [✔]
codon/triplet code/each set of three bases codes for an amino acid [✔]
This part was poorly answered by many candidates and was unable to state that genetic code is a sequence of bases in DNA and each codon codes for an amino acid. Often complex answers were written which were incorrect.