DP Chemistry (first assessment 2025)

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Question 21M.1A.SL.TZ1.20

Date	May 2021	Marks available	[Maximum mark: 1]	Reference code	21M.1A.SL.TZ1.20
Level	SL	Paper	1A	Time zone	TZ1
Command term	Solve	Question number	20	Adapted from	N/A

20.

[Maximum mark: 1]

21M.1A.SL.TZ1.20

Which solution has a pH of 9?

A. 1.0 × 10⁻⁹ mol dm⁻³ $HCl$ (aq)

B. 1.0 × 10⁻⁵ mol dm⁻³ $KOH$ (aq)

C. 1.0 × 10⁻⁹ mol dm⁻³ $KOH$ (aq)

D. 1.0 × 10⁻⁵ mol dm⁻³ $HCl$ (aq)

[1]

Markscheme

B

Syllabus sections

Reactivity 3. What are the mechanisms of chemical change? » Reactivity 3.1—Proton transfer reactions » Reactivity 3.1.4—The pH scale can be used to describe the [H+] of a solution: pH = –log10[H+]; [H+] = 10–pH. Perform calculations involving the logarithmic relationship between pH and [H+].

Reactivity 3. What are the mechanisms of chemical change? » Reactivity 3.1—Proton transfer reactions » Reactivity 3.1.5—The ion product constant of water, Kw, shows an inverse relationship between [H+] and [OH–]. Kw = [H+] [OH–]. Recognize solutions as acidic, neutral and basic from the relative values of [H+] and [OH–].