DP Chemistry (first assessment 2025)

Question 21N.2.SL.TZ0.5

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

[Maximum mark: 6]

21N.2.SL.TZ0.5

Phosphoric acid, H₃PO₄, can undergo stepwise neutralization, forming amphiprotic species.

(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 reason that sodium hydroxide is considered a Brønsted–Lowry base.

[1]

Markscheme

«OH⁻ is a» proton acceptor ✔

Syllabus sections