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Rates, equilibrium and pH

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Question 42

This question is about weak acids.

Benzoic acid, C6H5COOH\text{C}_6\text{H}_5\text{COOH}C6​H5​COOH, is a weak monobasic acid.

a.

Write the expression for the acid dissociation constant, KaK_aKa​, of C6H5COOH\text{C}_6\text{H}_5\text{COOH}C6​H5​COOH.

[1]
b.

The expression for the acid dissociation constant, KaK_aKa​, of C6H5COOH\text{C}_6\text{H}_5\text{COOH}C6​H5​COOH can be simplified to:

Ka=[H+]2[C6H5COOH] K_a = \frac{[\text{H}^+]^2}{[\text{C}_6\text{H}_5\text{COOH}]} Ka​=[C6​H5​COOH][H+]2​

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

[1]
c.

An analyst performs a titration preparation and measures the pKa\text{p}K_apKa​ of a sample of C6H5COOH\text{C}_6\text{H}_5\text{COOH}C6​H5​COOH at 298 K298\text{ K}298 K.

  • The concentration of C6H5COOH\text{C}_6\text{H}_5\text{COOH}C6​H5​COOH is 0.0250 mol dm−30.0250\text{ mol dm}^{-3}0.0250 mol dm−3.
  • The measured pH of the C6H5COOH\text{C}_6\text{H}_5\text{COOH}C6​H5​COOH solution is 2.912.912.91.

Use the simplified expression to calculate the pKa\text{p}K_apKa​ value of C6H5COOH\text{C}_6\text{H}_5\text{COOH}C6​H5​COOH. Give your answer to 2 decimal places.

[3]

Rates, equilibrium and pH Questions

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