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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	Formulate	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]

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

[2]

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]

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

[1]

Markscheme

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

Accept net ionic equation.

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.

«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.

«OH⁻ is a» proton acceptor ✔

Examiners report

[N/A]

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.
19M.2.sl.TZ1.2a:
Draw the structure of the conjugate base of benzoic acid showing all the atoms and all the bonds.
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

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