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Rates, equilibrium and pH

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Question 30

This question is about weak acids.

a.

Propanoic acid, CH3CH2COOH\text{CH}_3\text{CH}_2\text{COOH}CH3​CH2​COOH, is a weak monobasic acid.

Write the expression for the acid dissociation constant, KaK_aKa​, of CH3CH2COOH\text{CH}_3\text{CH}_2\text{COOH}CH3​CH2​COOH.

[1]
b.

The expression for the acid dissociation constant, KaK_aKa​, of CH3CH2COOH\text{CH}_3\text{CH}_2\text{COOH}CH3​CH2​COOH can be simplified to:

Ka=[H+]2[CH3CH2COOH] K_a = \frac{[\text{H}^+]^2}{[\text{CH}_3\text{CH}_2\text{COOH}]} Ka​=[CH3​CH2​COOH][H+]2​

State one approximation that allows the original expression to be simplified to this form.

[1]
c.

A student carries out an experiment to determine the pKa\text{p}K_apKa​ value of a solution of CH3CH2COOH\text{CH}_3\text{CH}_2\text{COOH}CH3​CH2​COOH.

  • The concentration of CH3CH2COOH\text{CH}_3\text{CH}_2\text{COOH}CH3​CH2​COOH is 0.150 mol dm−30.150\text{ mol dm}^{-3}0.150 mol dm−3.
  • The pH of CH3CH2COOH\text{CH}_3\text{CH}_2\text{COOH}CH3​CH2​COOH is 2.852.852.85.

Use the simplified expression to calculate the pKa\text{p}K_apKa​ value of CH3CH2COOH\text{CH}_3\text{CH}_2\text{COOH}CH3​CH2​COOH. Give your answer to 2 decimal places.

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Rates, equilibrium and pH Questions

  1. A Level
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  3. /Rates, equilibrium and pH