Sample pH / Buffer Problems

 

Common Biochemical Buffers

Buffer System

MW

pKa1

pKa2

pKa3

Formic acid (HCOOH)

46.02

3.85

 

 

Acetic Acid (CH3COOH)

60.04

4.76

 

 

Phosphoric (H3PO4)

97.97

2.2

6.86

12.4

Carbonic (H2CO3)

62.01

6.10

10.22

 

Tris (acid form)

121.1

8.15

 

 

 

 

(These are for your practice.  If you can do these, you should be able to do any I give you on the exams.)

 

 

1)     Starting with solid Tris base and 1M HCl, describe how one would make 2 liters of 200 mM (= 0.2 M) Tris-HCl buffer pH 7.5

 

 

 

 

2)   Starting with 5 M Phosphoric acid and 10 M NaOH, make 1000 ml of .25 M Na Phosphate buffer pH 7.

 

 

 

3)   Starting with 5 M Phosphoric acid and 10 M NaOH, make 1000 ml of .25 M Na Phosphate buffer pH 12.

 

 

 

4)   I have 500 ml of 0.2 M Na acetate buffer pH 4.0.  To this I add 500 ml of 0.2 M Na acetate (not a buffered solution, just plain old sodium acetate).  What is the pH of the resulting solution?  (Nowhere near as hard as it looks at first glance.)

 

 

 

5)   Draw the titration curve for the titration of 200 ml .05 M formic acid with 1 M potassium hydroxide.

 

 

 

6)   What is the pH of each of the following solutions:

      (a) .0.4 M phosphoric acid

      (b) mixture of 100 ml 1 M acetic acid and 10 ml 1 M Na acetate

      (c) mixture 0f 10 ml 1 M acetic acid and 100 ml 1 M Na acetate

      (d) .02 M HCl

      (e) mixture of 100 ml 1 M potassium formate and 30 ml 1 M formic acid