| Date | November 2016 | Marks available | 3 | Reference code | 16N.3.hl.TZ0.13 |
| Level | HL | Paper | 3 | Time zone | TZ0 |
| Command term | Determine | Question number | 13 | Adapted from | N/A |
Question
Amino acids are usually identified by their common names. Use section 33 of the data booklet.
Amino acids act as buffers in solution. In aspartic acid, the side chain (R group) carboxyl has pKa = 4.0. Determine the percentage of the side chain carboxyl that will be ionized (–COO–) in a solution of aspartic acid with pH = 3.0. Use section 1 of the data booklet.
Markscheme
«–COOH \( \rightleftharpoons \) –COO– + H+ (–COOH = HA ; –COO– = A–)»
pH = pKa + log \(\frac{{\left[ {{{\rm{A}}^ - }} \right]}}{{\left[ {{\rm{HA}}} \right]}}\) / 3.0 = 4.0 + log \(\frac{{\left[ { - {\rm{CO}}{{\rm{O}}^ - }} \right]}}{{\left[ { - {\rm{COOH}}} \right]}}\) / –1.0 = log \(\frac{{\left[ { - {\rm{CO}}{{\rm{O}}^ - }} \right]}}{{\left[ { - {\rm{COOH}}} \right]}}\)
10–1=\(\frac{{\left[ { - {\rm{CO}}{{\rm{O}}^ - }} \right]}}{{\left[ { - {\rm{COOH}}} \right]}}\)
«percentage ionized/–COO– = \(\frac{1}{{1 + 10}}\) × 100 =» 9.1 «%»
Award [3] for correct final answer.
