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Date	May 2019	Marks available	2	Reference code	19M.2.sl.TZ1.2
Level	SL	Paper	2	Time zone	TZ1
Command term	Formulate	Question number	2	Adapted from	N/A

Question

Benzoic acid, C₆H₅COOH, is another derivative of benzene.

Draw the structure of the conjugate base of benzoic acid showing all the atoms and all the bonds.

[1]

The pH of an aqueous solution of benzoic acid at 298 K is 2.95. Determine the concentration of hydroxide ions in the solution, using section 2 of the data booklet.

[2]

b(i).

Formulate the equation for the complete combustion of benzoic acid in oxygen using only integer coefficients.

[2]

b(ii).

Suggest how benzoic acid, M_r = 122.13, forms an apparent dimer, M_r = 244.26, when dissolved in a non-polar solvent such as hexane.

[1]

Markscheme

[✔]

Note: Accept Kekulé structures.

Negative sign must be shown in correct position- on the O or delocalised over the carboxylate.

ALTERNATIVE 1:
[H⁺] «= 10^−2.95» = 1.122 × 10⁻³ «mol dm⁻³» [✔]

«[OH⁻] = $\frac{{1.00 \times {{10}^{ - 14}}{\text{ mo}}{{\text{l}}^2}{\text{ d}}{{\text{m}}^{ - 6}}}}{{1.22 \times {{10}^{ - 3}}{\text{ mol d}}{{\text{m}}^{ - 3}}}}$ =» 8.91 × 10⁻¹² «mol dm⁻³» [✔]

ALTERNATIVE 2:
pOH = «14 − 2.95 =» 11.05 [✔]

«[OH⁻] = 10^−11.05 =» 8.91 × 10⁻¹² «moldm⁻³» [✔]

Note: Award [2] for correct final answer.

Accept other methods.

b(i).

2C₆H₅COOH(s) + 15O₂ (g) → 14CO₂ (g) + 6H₂O(l)

correct products [✔]

correct balancing [✔]

b(ii).

«intermolecular» hydrogen bonding [✔]

Note: Accept diagram showing hydrogen bonding.

Examiners report

Most failed to score a mark for the conjugate base of benzoic acid as either they didn’t show all bonds and atoms in the ring and/or they did not put the minus sign in the correct place. Some didn't read the question carefully so gave the structure of the acid form.

Many students could correctly calculate the hydroxide concentration, but some weaker students calculated hydrogen ion concentration only.

b(i).

Most students earned at least one mark for writing the correct products of the combustion of benzoic acid but the balancing appeared to be difficult for some.

b(ii).

Very few students answered this question correctly, thinking benzoic would bond with the hexane even though it was a non-polar solvent. It was very rare for a student to realize there was intermolecular hydrogen bonding.

Syllabus sections

Core » Topic 1: Stoichiometric relationships » 1.1 Introduction to the particulate nature of matter and chemical change

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