Buffers. CH 3 COOH(aq) CH 3 COO¯(aq) + H + (aq) NH 3 (aq) + H 2 O(l) OH¯(aq) + NH 4 + (aq)

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Presentation transcript:

Buffers

CH 3 COOH(aq) CH 3 COO¯(aq) + H + (aq) NH 3 (aq) + H 2 O(l) OH¯(aq) + NH 4 + (aq)

Calculating the pH of an acidic buffer solution Calculate the pH of a buffer whose [HA] is 0.1 mol dm -3 and [A¯] of 0.1 mol dm -3. K a = [H + (aq)] [A¯(aq)] [HA(aq)] re-arrange [H + (aq)] = [HA(aq)] x K a [A¯(aq)]

Calculating the pH of an acidic buffer solution Calculate the pH of a buffer whose [HA] is 0.1 mol dm -3 and [A¯] of 0.1 mol dm -3. K a = [H + (aq)] [A¯(aq)] [HA(aq)] re-arrange [H + (aq)] = [HA(aq)] x K a [A¯(aq)] from information given[A¯] = 0.1 mol dm -3 [HA] = 0.1 mol dm -3

Calculating the pH of an acidic buffer solution Calculate the pH of a buffer whose [HA] is 0.1 mol dm -3 and [A¯] of 0.1 mol dm -3. K a = [H + (aq)] [A¯(aq)] [HA(aq)] re-arrange [H + (aq)] = [HA(aq)] x K a [A¯(aq)] from information given[A¯] = 0.1 mol dm -3 [HA] = 0.1 mol dm -3 If the K a of the weak acid HA is 2 x mol dm -3. [H + (aq)]= 0.1 x 2 x = 2 x mol dm

Calculating the pH of an acidic buffer solution Calculate the pH of a buffer whose [HA] is 0.1 mol dm -3 and [A¯] of 0.1 mol dm -3. K a = [H + (aq)] [A¯(aq)] [HA(aq)] re-arrange [H + (aq)] = [HA(aq)] x K a [A¯(aq)] from information given[A¯] = 0.1 mol dm -3 [HA] = 0.1 mol dm -3 If the K a of the weak acid HA is 2 x mol dm -3. [H + (aq)]= 0.1 x 2 x = 2 x mol dm pH= - log 10 [H + (aq)] = 3.699

Calculating the pH of an acidic buffer solution Calculate the pH of the solution formed when 500cm 3 of 0.1 mol dm -3 of weak acid HX is mixed with 500cm 3 of a 0.2 mol dm -3 solution of its salt NaX. K a = 4 x mol dm -3.

Calculating the pH of an acidic buffer solution Calculate the pH of the solution formed when 500cm 3 of 0.1 mol dm -3 of weak acid HX is mixed with 500cm 3 of a 0.2 mol dm -3 solution of its salt NaX. K a = 4 x mol dm -3. K a = [H + (aq)] [X¯(aq)] [HX(aq)]

Calculating the pH of an acidic buffer solution Calculate the pH of the solution formed when 500cm 3 of 0.1 mol dm -3 of weak acid HX is mixed with 500cm 3 of a 0.2 mol dm -3 solution of its salt NaX. K a = 4 x mol dm -3. K a = [H + (aq)] [X¯(aq)] [HX(aq)] re-arrange[H + (aq)] = [HX(aq)] K a [X¯(aq)]

Calculating the pH of an acidic buffer solution Calculate the pH of the solution formed when 500cm 3 of 0.1 mol dm -3 of weak acid HX is mixed with 500cm 3 of a 0.2 mol dm -3 solution of its salt NaX. K a = 4 x mol dm -3. K a = [H + (aq)] [X¯(aq)] [HX(aq)] re-arrange[H + (aq)] = [HX(aq)] K a [X¯(aq)] The solutions have been mixed; the volume is now 1 dm 3 therefore [HX] = 0.05 mol dm -3 and [X¯] = 0.10 mol dm -3

Calculating the pH of an acidic buffer solution Calculate the pH of the solution formed when 500cm 3 of 0.1 mol dm -3 of weak acid HX is mixed with 500cm 3 of a 0.2 mol dm -3 solution of its salt NaX. K a = 4 x mol dm -3. K a = [H + (aq)] [X¯(aq)] [HX(aq)] re-arrange[H + (aq)] = [HX(aq)] K a [X¯(aq)] The solutions have been mixed; the volume is now 1 dm 3 therefore [HX] = 0.05 mol dm -3 and [X¯] = 0.10 mol dm -3 Substituting[H + (aq)] = 0.05 x 4 x = 2 x mol dm pH= - log 10 [H + (aq)] = 4.699

4.3 exercise 3 - Buffers Homework: page 40-51, Questions 2, 5 and 6.