2. Buffers Calculating the pH of a buffer solution

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

4. Step 1 Write the expression for K a. HA(aq) H + (aq) + A - (aq) K a = [H + (aq)][A - (aq)]/ [HA(aq)]

5. K a = [H + (aq)][A - (aq)] [HA(aq)]

6. 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

7. Step 3 Write the new expression for K a K a = [H + (aq)][salt(aq)] [acid]

8. 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 )

9. 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 !