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Calculating the pH of a buffer solution

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1 Calculating the pH of a buffer solution

2 Worked example A buffer solution is made from 0.1moldm-3 ethanoic acid(CH3COOH) and 0.2moldm-3 sodium ethanoate(CH3COONa). Calculate the pH of this buffer solution. (Ka for ethanoic acid is 1.7x 10-5moldm-3 at 298K)

3 Step 1 Write the expression for Ka. HA(aq) H+(aq) + A-(aq) Ka= [H+(aq)][A-(aq)]/ [HA(aq)]

4 Ka= [H+(aq)][A-(aq)] [HA(aq)]

5 Step 2 Apply the 2 assumptions
* All the A-(aq) comes from the salt i.e. [A-(aq) ] = [salt(aq)] * Almost all the HA molecules remain unchanged i.e. [HA(aq)]equilibrium = [HA(aq)]initial

6 Step 3 Write the new expression for Ka
Ka = [H+(aq)][salt(aq)] [acid]

7 Step 4 put the values given in the new expression to find [H+(aq)]
1.7 x 10-5 = [H+(aq)] x 0.2/0.1 [H+(aq)]= 1.7 x 10-5 x 0.1/0.2=8.5x10-6(moldm-3)

8 Step 5 Work out the pH pH = - log[H+(aq)] = -log( 8.5x 10-6) = 5.07 (at 298K) That’s it –Chemistry is fun !

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