DP Chemistry (first assessment 2025)

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Question EXM.2.HL.TZ0.2a

Date	May Example question	Marks available	[Maximum mark: 3]	Reference code	EXM.2.HL.TZ0.2a
Level	HL	Paper	2	Time zone	TZ0
Command term	Determine	Question number	a	Adapted from	N/A

a.

[Maximum mark: 3]

EXM.2.HL.TZ0.2a

(a)

Determine the ratio in which 0.1 mol dm^–3 NaH₂PO₄ and 0.1 mol dm^–3 Na₂HPO₄ should be mixed to obtain a buffer with pH= 7.8.

pK_a NaH₂PO₄ = 7.20

[3]

Markscheme

$pH = p K_{a} + \log \frac{[{HPO}_{4}^{2 -}]}{[H_{2} {PO}_{4}^{-}]} / 7.80 = 7.20 + \log \frac{[{HPO}_{4}^{2 -}]}{[H_{2} {PO}_{4}^{-}]}$

$\log \frac{[{HPO}_{4}^{2 -}]}{[H_{2} {PO}_{4}^{-}]} =$ « $7.80 - 7.20 =$ » 0.6 ✓

$\frac{[{HPO}_{4}^{2 -}]}{[H_{2} {PO}_{4}^{-}]} =$ « $10^{0.6} =$ » 3.98 ✓

Na₂PO₄ to Na₂HPO₄ = 3.98:1 ✓

Syllabus sections

Reactivity 3. What are the mechanisms of chemical change?

Reactivity 3. What are the mechanisms of chemical change? » Reactivity 3.1—Proton transfer reactions » Reactivity 3.1.17—The pH of a buffer solution depends on both: • the pKa or pKb of its acid or base • the ratio of the concentration of acid or base to the concentration of the conjugate base or acid. Solve problems involving the composition and pH of a buffer solution, using the equilibrium constant.

Reactivity 3. What are the mechanisms of chemical change? » Reactivity 3.1—Proton transfer reactions