DP Chemistry (last assessment 2024)

Question 18M.3.hl.TZ2.8

Date	May 2018	Marks available	[Maximum mark: 6]	Reference code	18M.3.hl.TZ2.8
Level	hl	Paper	3	Time zone	TZ2
Command term	Calculate, Draw, Estimate, Sketch	Question number	8	Adapted from	N/A

[Maximum mark: 6]

18M.3.hl.TZ2.8

Amino acids are the building blocks of proteins.

(c)

Draw the structures of the main form of glycine in buffer solutions of pH 1.0 and 6.0.

The pK_a of glycine is 2.34.

[2]

Markscheme

M18/4/CHEMI/HP3/ENG/TZ2/08.c/M

Penalize charge on incorrect atom once only.

Penalize missing hydrogens or incorrect bond connectivities once only in Option B.

Accept condensed structural formulas.

Accept skeletal structures.

[2 marks]

(d)

Calculate the pH of a buffer system with a concentration of 1.25 × 10⁻³ mol dm⁻³ carbonic acid and 2.50 × 10⁻² mol dm⁻³ sodium hydrogen carbonate. Use section 1 of the data booklet.

pK_a (carbonic acid) = 6.36

[1]

Markscheme

ALTERNATIVE 1

«pH = 6.36 + log $\left( {\frac{{2.50 \times {{10}^{ - 2}}}}{{1.25 \times {{10}^{ - 3}}}}} \right)$ =»

7.66

ALTERNATIVE 2

«K_a= 4.4 × 10^–7 = [H⁺] $\left( {\frac{{2.50 \times {{10}^{ - 2}}}}{{1.25 \times {{10}^{ - 3}}}}} \right)$ , [H⁺] = 2.2 × 10^–8 mol dm^–3»

«pH =» 7.66

Do not accept “«pH =» 8”.

[1 mark]

(e)

Sketch the wedge and dash (3-D) representations of alanine enantiomers.

[1]

Markscheme

Penalize missing hydrogens or incorrect bond connectivities once only in Option B.

Wedges AND dashes must be used.

[1 mark]

(f)

UV-Vis spectroscopy can be used to determine the unknown concentration of a substance in a solution.

Calculate the concentration of an unknown sample of pepsin with an absorbance of 0.725 using section 1 of the data booklet.

Cell length = 1.00 cm

Molar absorptivity (extinction coefficient) of the sample = 49650 dm³ cm⁻¹ mol⁻¹

[1]

Markscheme

« $\frac{{0.725}}{{49650{\text{ d}}{{\text{m}}^3}\;{\text{c}}{{\text{m}}^{ - 1}}\;{\text{mo}}{{\text{l}}^{ - 1}} \times 1.00\,{\text{cm}}}} =$ » 1.46 × 10⁻⁵ «mol dm⁻³»