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CAIE A-Level Chemistry 25.1.3 pH, Ka, pKa and Kw

Practise stating the mathematical definitions of pH, Ka, pKa and Kw and rearranging them in calculations.

Syllabus
2028–2030
Course
Chemistry 9701
Level
A2

Exam points

  • use pH = −log10[H+], pKa = −log10Ka and the corresponding inverse powers of ten
  • write Ka from equilibrium concentrations and Kw = [H+][OH−] with the correct units
  • avoid introducing Kb or Kw=KaKb because those relationships are outside this stated objective

25.1.3—Math definitions: pH, Ka, pKa and Kw and use question 1

[Maximum number: 4]

Question (a)

(a)

The KaK_{a} of propanoic acid, CH3CH2COOH\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH}, is 1.35×10−5moldm−31.35 \times 10^{-5} \mathrm{moldm}^{-3} at 298 K.

Solution C is a solution of CH3CH2COOH\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH} with a pH of 3.60 at 298 K.

[ 4 ]

Question (i)

(i)

Calculate the concentration of hydroxide ions in solution C.

[OH−]=.............................. mol dm−3\left[\mathrm{OH}^{-}\right]=..............................\ \mathrm{mol\ dm}^{-3}
[ 1 ]

Question (ii)

(ii)

Table 3.1 shows three possible values of the KaK_{a} of dimethylpropanoic acid, (CH3)3CCOOH\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCOOH}.

Place a tick in Table 3.1 to show the correct value. Explain your answer.

Table 3.1

Table 3.1

explanation

[ 3 ]
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