Q4.

This question is about acidic solutions.

(a)

The acid dissociation constant, Ka, for ethanoic acid is given by the expression

[CH₂COO-][H+]

[CH₂COOH]

Ka

-3

The value of K₂ for ethanoic acid is 1.74 x 10-5 mol dm-³ at 25 °C

A buffer solution with a pH of 3.87 was prepared using ethanoic acid and sodium

ethanoate. In the buffer solution, the concentration of ethanoate ions was

0.136 mol dm¯³

Calculate the concentration of the ethanoic acid in the buffer solution.

Give your answer to three significant figures.

Concentration of acid

-

mol dm 3

(b)

In a different buffer solution, the concentration of ethanoic acid was 0.260 mol dm-³

and the concentration of ethanoate ions was 0.121 mol dm¯

-3

A 7.00 × 10-3 mol sample of sodium hydroxide was added to 500 cm³ of this buffer

solution.

Calculate the pH of the buffer solution after the sodium hydroxide was added.

Give your answer to two decimal places.

(3)/n(C)

A buffer solution with a pH of 4.5 is made by dissolving x g of sodium propanoate (C₂H5

COONa) in a solution of propanoic acid. The final volume of the buffer solution is 500 cm³, and

the final concentration of the propanoic acid is 0.50 mol dm-3 .The value of K₂= 1.74 × 10-5

mol dm-3 at 25 °C