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Date November 2021 Marks available 2 Reference code 21N.2.sl.TZ0.5
Level SL Paper 2 Time zone TZ0
Command term Calculate Question number 5 Adapted from N/A

Question

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

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

[1]
a.

Formulate two equations to show the amphiprotic nature of H2PO4.

[2]
b.

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]
c.

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

[1]
d.

Markscheme

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


Accept net ionic equation.

a.

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.

b.

«NaOH 28.40cm31000×0.5000moldm-3=0.01420mol»

«0.01420mol3=» 0.004733 «mol» ✔

«0.004733mol25.00cm31000=» 0.1893 «mol dm−3» ✔


Award [2] for correct final answer.

c.

«OH is a» proton acceptor ✔

d.

Examiners report

[N/A]
a.
[N/A]
b.
[N/A]
c.
[N/A]
d.

Syllabus sections

Core » Topic 1: Stoichiometric relationships » 1.3 Reacting masses and volumes
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Core » Topic 1: Stoichiometric relationships
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