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Edexcel IAL Chemistry 14.2 Conjugate acid–base pairs

Match formulas before labels: removing H⁺ from an acid gives its conjugate base, while adding H⁺ to a base gives its conjugate acid with the charge adjusted by one.

Syllabus
First assessment 2019
Course
Chemistry YCH11
Level
A2

Exam points

  • Complete proton-transfer equations and label the two conjugate acid–base pairs consistently.
  • Obtain a conjugate base by removing one H⁺ or a conjugate acid by adding one H⁺.
  • Reject proposed pairs whose formulas differ by more than one proton or another atom.

14.2—Identify Brønsted–Lowry conjugate acid-base pairs question 1

[Maximum number: 2]

This question is about acids and bases.

Some KaK_{\mathrm{a}} and pKa\mathrm{p} K_{\mathrm{a}} values for several weak acids are shown.

Table for Question 14.2—Identify Brønsted–Lowry conjugate acid-base pairs question 1 — Edexcel A-Level Chemistry A2

When methanoic acid is added to propanoic acid, an equilibrium is set up containing two acid-base pairs.

Complete the equilibrium, labelling the acid-base pairs as A1, B1 and A2, B2.

CH3CH2COOH+HCOOH⇌………………………………………………….………………………………………………………………..\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH}+\mathrm{HCOOH} \rightleftharpoons \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots . \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots . .

*(iv) A student measured the pH of separate 0.050 moldm−30.050 \mathrm{~mol} \mathrm{dm}^{-3} solutions of CH2ClCOOH\mathrm{CH}_{2} \mathrm{ClCOOH} and of CHCl2COOH\mathrm{CHCl}_{2} \mathrm{COOH}, using a pH meter.

The student also calculated the pH of these solutions, making two assumptions:
- [H+]=[A−]\left[\mathrm{H}^{+}\right]=\left[\mathrm{A}^{-}\right]
- [HA]equilibrium =[HA]initial [\mathrm{HA}]_{\text {equilibrium }}=[\mathrm{HA}]_{\text {initial }}

The measured and calculated pH values are shown.

Table for Question 14.2—Identify Brønsted–Lowry conjugate acid-base pairs question 1 — Edexcel A-Level Chemistry A2

Discuss the differences between the student's measured and calculated pH values.

In your answer, you should
- show how the student calculated their pH values

[Ka(CH2ClCOOH)=1.4×10−3 moldm−3;Ka(CHCl2COOH)=4.5×10−2 moldm−3]\begin{aligned} & {\left[K_{\mathrm{a}}\left(\mathrm{CH}_{2} \mathrm{ClCOOH}\right)=1.4 \times 10^{-3} \mathrm{~mol} \mathrm{dm}^{-3} ;\right.} \\ & \left.K_{\mathrm{a}}\left(\mathrm{CHCl}_{2} \mathrm{COOH}\right)=4.5 \times 10^{-2} \mathrm{~mol} \mathrm{dm}^{-3}\right] \end{aligned}

- explain, with reference to the assumptions made, why there is a difference between the calculated and measured pH values
- suggest why the measured pH values are higher than the calculated pH values.

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