Question

1. Which of the following aqueous solutions are good buffer systems?

0.24 M hydrobromic acid + 0.21 M potassium bromide

0.33 M ammonium bromide + 0.37 M ammonia

0.39 M hydrocyanic acid + 0.23 M potassium cyanide

0.14 M acetic acid + 0.21 M sodium acetate

0.15 M sodium fluoride + 0.23 M hydrofluoric acid

2. Which of the following aqueous solutions are good buffer systems?

0.34 M ammonia + 0.34 M ammonium nitrate

0.16 M barium hydroxide + 0.23 M barium bromide

0.26 M hydrobromic acid + 0.22 M sodium bromide

0.12 M acetic acid + 0.14 M potassium acetate

0.39 M calcium iodide + 0.29 M calcium nitrate

3. A solution contains 0.474 M sodium acetate and 0.434 M acetic acid.  

The pH of this solution is ____

4.A solution contains 0.189 M potassium acetate and 0.151 M acetic acid.

The pH of this solution is ____

Answer 1

1.

Answer is 0.14 M acetic acid + 0.21 M sodium acetate

(Since its a weak acid and its conjugated base).

2.

0.12 M acetic acid + 0.14 M potassium acetate

(Since its a weak acid and its conjugated base).

3.

Using Hender-Hesselbalach equation

pH = pKa + log { [salt] / [acid] }

pH = pKa + log { [CH3COONa] / [CH3COOH] }

pKa of CH3COOH = 4.75

So,

pH = 4.75 + log(0.474 / 0.434)

   = 4.75 + log(1.09)

     = 4.75 + 0.04

     = 4.79

4.

Using Hender-Hesselbalach equation

pH = pKa + log { [salt] / [acid] }

pH = pKa + log { [CH3COOK] / [CH3COOH] }

pKa of CH3COOH = 4.75

So,

pH = 4.75 + log(0.189 / 0.151)

   = 4.75 + log(1.25)

     = 4.75 + 0.10

     = 4.85