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Date	May 2019	Marks available	1	Reference code	19M.2.sl.TZ1.2
Level	SL	Paper	2	Time zone	TZ1
Command term	Draw	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 8: Acids and bases » 8.1 Theories of acids and bases

Show 47 related questions

18M.1.sl.TZ1.19:
Which classification is correct for the reaction?

H₂PO₄⁻(aq) + H₂O(l) → HPO₄²⁻(aq) + H₃O⁺(aq)
22M.2.hl.TZ1.4a: State the relationship between NH4+ and NH3 in terms of the Brønsted–Lowry theory.
18N.2.hl.TZ0.6a.i:
State the equation for the reaction of each substance with water.
17N.2.sl.TZ0.5b.ii: State what is meant by the term conjugate base.
17N.2.sl.TZ0.5b.iii: State the conjugate base of the hydroxide ion, OH–.
17N.2.sl.TZ0.5b.i: Identify two different amphiprotic species in the above reactions.
17M.2.sl.TZ2.7a.ii:
Identify one conjugate acid-base pair in the reaction.
22M.2.sl.TZ2.3c(ii): State the meaning of a strong Brønsted–Lowry acid.
17M.1.sl.TZ1.19: Which is an acid-base conjugate pair? A. H3O+ / OH– B. H2SO4 / SO42– C. ...
17M.1.hl.TZ1.24: Which species produced by the successive dissociations of phosphoric acid, H3PO4,...
16N.1.sl.TZ0.19: Which species behave as Brønsted–Lowry bases in the following reaction? H2SO4 + HNO3...
17M.2.sl.TZ2.7a.i:
Identify the amphiprotic species.
19N.2.sl.TZ0.4a(i): Identify a conjugate acid–base pair in the equation.
18M.1.hl.TZ1.24: What describes HPO42−? A. Amphiprotic but not amphoteric B. Amphoteric but not...
18M.2.hl.TZ1.5c:
Write an equation to show ammonia, NH₃, acting as a Brønsted–Lowry base and a different equation to show it acting as a Lewis base.
22M.2.hl.TZ2.6d(ii): State the meaning of a strong Brønsted–Lowry acid.
18N.2.sl.TZ0.6a:
State the equation for the reaction of each substance with water.
21N.2.sl.TZ0.5a:
Formulate an equation for the reaction of one mole of phosphoric acid with one mole of sodium hydroxide.
21N.2.sl.TZ0.5d: Outline the reason that sodium hydroxide is considered a Brønsted–Lowry base.
17M.1.hl.TZ1.26:
Which species acts as a Lewis and Brønsted–Lowry base?

A. [Al(H₂O)₆]³⁺

B. BF₃

C. NH₄⁺

D. OH⁻
19M.2.sl.TZ2.5a(ii):
The hydrogencarbonate ion, produced in Equilibrium (2), can also act as an acid.

State the formula of its conjugate base.
17M.1.sl.TZ2.20: Which of the following is correct? A. A weak acid is a proton donor and its aqueous...
19N.2.hl.TZ0.4a(i): Identify a conjugate acid–base pair in the equation.
19N.3.sl.TZ0.1c(i): Justify this hypothesis.
17N.2.sl.TZ0.5c:
A student working in the laboratory classified HNO₃, H₂SO₄, H₃PO₄ and HClO₄ as acids based on their pH. He hypothesized that “all acids contain oxygen and hydrogen”.

Evaluate his hypothesis.
21N.2.hl.TZ0.5a:
Formulate an equation for the reaction of one mole of phosphoric acid with one mole of sodium hydroxide.
17M.2.sl.TZ2.7b.i:
Calculate the amount, in mol, of NaOH(aq) used.
19N.1.sl.TZ0.21: Which is an example of an amphiprotic species? A. Al2O3 B. CO32− C. P4O10 D. HPO42−
19N.1.sl.TZ0.20: What is the difference between a conjugate Brønsted–Lowry acid–base pair? A. Electron...
18N.1.sl.TZ0.19:
Which two species act as Brønsted–Lowry acids in the reaction?

H₂PO₄⁻ (aq) + OH⁻ (aq) $\rightleftharpoons$ HPO₄²⁻ (aq) + H₂O (l)

A. HPO₄²⁻ (aq) and OH⁻ (aq)

B. H₂PO₄⁻ (aq) and HPO₄²⁻ (aq)

C. HPO₄²⁻ (aq) and H₂O (l)

D. H₂PO₄⁻ (aq) and H₂O (l)
20N.2.hl.TZ0.5d:
Calculate the $K_{b}$ of the conjugate base of ethanoic acid using sections 2 and 21 of the data booklet.
19M.2.hl.TZ2.5a(ii):
The hydrogencarbonate ion, produced in Equilibrium (2), can also act as an acid.

State the formula of its conjugate base.
18M.2.sl.TZ2.2d:
Outline why H₃PO₄/HPO₄²⁻ is not a conjugate acid-base pair.
19M.2.hl.TZ2.5e:
The reaction of the hydroxide ion with carbon dioxide and with the hydrogencarbonate ion can be represented by Equations 3 and 4.

Equation (3) OH⁻ (aq) + CO₂ (g) → HCO₃⁻ (aq)
Equation (4) OH⁻ (aq) + HCO₃⁻ (aq) → H₂O (l) + CO₃²⁻ (aq)

Discuss how these equations show the difference between a Lewis base and a Brønsted–Lowry base.

Equation (3):

Equation (4):
21N.2.hl.TZ0.5d: Outline the reasons that sodium hydroxide is considered a Brønsted–Lowry and Lewis base.
21M.1.hl.TZ1.26:
Which is a Lewis acid, but not a Brønsted-Lowry acid?

A.   ${BF}_{3}$

B.   $H_{3} O^{+}$

C.   ${NH}_{3}$

D.   ${Cl}^{-}$
19M.2.hl.TZ1.2d:
Draw the structure of the conjugate base of benzoic acid showing all the atoms and all the bonds.
21M.1.sl.TZ2.19: Which cannot act as a Brønsted–Lowry base? A. HPO42− B. H2O C. CH4 D. NH3
21M.2.sl.TZ1.2b(i): State the formula of its conjugate base.
20N.1.sl.TZ0.20:
Which of these acids has the weakest conjugate base?

A.   $HCl$

B.   ${CH}_{3} COOH$

C.   ${NH}_{4} Cl$

D.   $C_{6} H_{5} COOH$
20N.2.sl.TZ0.1c(ii):
State the formula of the conjugate base of hypochlorous acid.
20N.2.hl.TZ0.1c(ii):
State the formula of the conjugate base of hypochlorous acid.
20N.1.hl.TZ0.26:
Which species is a Lewis acid but not a Brønsted–Lowry acid?

A.   ${Cu}^{2 +}$

B.   ${NH}_{4}^{+}$

C.   $Cu$

D.   ${CH}_{3} COOH$
22M.2.sl.TZ1.2e(i): State the relationship between NH4+ and NH3 in terms of the Brønsted–Lowry theory.
21N.2.hl.TZ0.5b:
Formulate two equations to show the amphiprotic nature of H₂PO₄⁻.
21N.1.sl.TZ0.21: What is the conjugate acid of HS−? A. H2S B. S2− C. H2SO3 D. H2SO4
21N.2.sl.TZ0.5b:
Formulate two equations to show the amphiprotic nature of H₂PO₄⁻.

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