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

Question

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

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

«NaOH28.40cm31000×0.5000moldm-3=0.01420mol»

«0.01420mol3=» 0.004733 «mol» ✔

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

 

Award [2] for correct final answer.

c.

Brønsted–Lowry base:
proton acceptor

AND

Lewis Base:
e pair donor/nucleophile ✔

d.

Examiners report

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

Syllabus sections

Core » Topic 8: Acids and bases » 8.1 Theories of acids and bases
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