DP Chemistry (first assessment 2025)

Question 21N.2.HL.TZ0.5

Date	November 2021	Marks available	[Maximum mark: 6]	Reference code	21N.2.HL.TZ0.5
Level	HL	Paper	2	Time zone	TZ0
Command term	Calculate, Formulate, Outline	Question number	5	Adapted from	N/A

[Maximum mark: 6]

21N.2.HL.TZ0.5

Phosphoric acid, H₃PO₄ can form three different salts depending on the extent of neutralisation by sodium hydroxide.

(a)

Formulate an equation for the reaction of one mole of phosphoric acid with one mole of sodium hydroxide.

[1]

Markscheme

H₃PO₄(aq) + NaOH (aq) → NaH₂PO₄(aq) + H₂O (l) ✔

Accept net ionic equation.

(b)

Formulate two equations to show the amphiprotic nature of H₂PO₄⁻.

[2]

Markscheme

H₂PO₄⁻(aq) + H⁺(aq) → H₃PO₄(aq) ✔

H₂PO₄⁻(aq) + OH⁻(aq) → HPO₄²⁻(aq) + H₂O (l) ✔

Accept reactions of H₂PO₄⁻ with any acidic, basic or amphiprotic species, such as H₃O⁺, NH₃ or H₂O.

Accept H₂PO₄⁻ (aq) → HPO₄²⁻ (aq) + H⁺ (aq) for M2.

(c)

Calculate the concentration of H₃PO₄ if 25.00 cm³ is completely neutralised by the addition of 28.40 cm³ of 0.5000 mol dm⁻³ NaOH.

[2]

Markscheme

« $NaOH \frac{28.40 {cm}^{3}}{1000} \times 0.5000 mol {dm}^{- 3} = 0.01420 mol$ »

« $\frac{0.01420 mol}{3} =$ » 0.004733 «mol» ✔

« $\frac{0.004733 mol}{\frac{25.00 {cm}^{3}}{1000}} =$ » 0.1893 «mol dm⁻³» ✔

Award [2] for correct final answer.

(d)

Outline the reasons that sodium hydroxide is considered a Brønsted–Lowry and Lewis base.

[1]

Markscheme

Brønsted–Lowry base:
proton acceptor

AND

Lewis Base:
e^– pair donor/nucleophile ✔

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