Make a chart of pH when 1 M solutions of monobasic and dibasic potassium phospha

Question

Make a chart of pH when 1 M solutions of monobasic and dibasic potassium phosphate are diluted 50-fold by adding the following ml to 490 ml of water in the following combination. The pKa for    the reaction is 7.2.

           monobasic K+ phosphate      dibasic K+ phosphate   pH

               1.0 ml 9.0 ml

               2.0 ml 8.0 ml

               3.0 ml 7.0 ml

               4.0 ml 6.0 ml

               5.0 ml 5.0 ml

               6.0 ml 4.0 ml

               7.0 ml 3.0 ml

               8.0 ml 2.0 ml

               9.0 ml 1.0 ml

Explanation / Answer

Mixture of dibasic and monobasic acid will act as a buffer solution. In solution dibasic acid will act as the salt of the monobasic acid. pH can be measured using Henderson's equation, pH = pKa = log[salt] / [acid].

Here, pH= 7.2 + log[dibasic acid] /[ monobasic acid]

1. pH = 7.2 + log (9×1/500)/ (1×1/500) = 7.2 + log (9/500) / (1/500) = 7.2 + log 9/1 = 8.15

2. pH = 7.2 + log 8/2 = 7.8

3. pH = 7.2 + log 7/3 = 7.5

Apply same method, the answers will be..

4. pH = 7.37

5. pH = 7.2

6. pH= 7.02

7. pH= 6.8

8. pH= 6.6

9.pH = 6.2

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