Buffer Solution

Experiment Write-Up Name: Prakhar Birla Grade: 12 C Subject: Chemistry HL Aim → To verify the addition of an acid or alkali to a buffer solution to have negligible effect on the pH of the buffer solution, and hence to verify the validity of the solution to be a buffer. Hypothesis → Buffers are solutions which resist changes in pH on addition of an acid or alkali. A buffer solution usually consists of a: • weak acid in presence of one of its salt or • weak base in presence of one of its salt In our case it is ‘a weak acid in presence of one of its salt’ i.e. CH 3 COOH in presence of CH 3 COONa . Partly

ˆ ˆˆ ˆˆ ˆˆ ˆˆ ˆˆˆ † CH 3 COO   H  CH 3 COOH ‡ Dissociated CH 3 COONa 





  CH 3 COO   Na 

Fully Dissocited

Hence the mixture contains a relatively high concentration of CH 3 COOH (an acid) and a relatively high concentration of CH 3 COO  (a base). So when an acid is added to this mixture the H  ions in the acid combine with CH 3 COO  to form CH 3 COOH . As there is a large concentration of CH 3 COO  ions, nearly all H  ions are removed, thus [ H  ] changes are very little and the pH is only slightly altered. Apparatus → 1. Concentrated CH 3 COOH 2. 3. 4. 5. 6. 7.

Solid CH 3 COONa Distilled Water Weighing Machine Measuring Cylinder Beakers pH Meter

Procedure → 1. Calibrate the pH Meter. 2. Calculate and prepare the right concentration of CH 3 COOH and CH 3 COONa for making 200 cm3 of each i.e. 400 cm3 in total. 3. In a separate beaker, slowly add the acid into the base. 4. Introduce the pH Meter into the solution to note the initial pH of the obtained buffer. 5. Separate this mixture (400 cm3) into 3 beakers and label them as follows a. 100 cm3

6. 7. 8. 9.

b. 100 cm3 c. 200 cm3 In solution (a) introduce a pH meter and note then initial pH of the solution, and then add 1 cm3 at a time of 2M HCl until there is a significant change in the pH of the solution. Record your observations. Repeat steps 6-7 for solution (b) with 1M of NaOH . Repeat steps 6-7 for solution (c) with 2M of HCl .

Data Collection → Buffer solution (a) i.e. 50 cm3 CH 3 COOH and 50 cm3 CH 3 COONa Sr. No. Total Volume of 2M HCl Resultant added pH 1 0 4.8 2 1 4.7 3 2 4.6 4 3 4.6 5 4 4.6 6 5 4.5 7 6 4.5

Series 1 f(x)=-0.057142857*x+4.8142857; R²=0.9412

y 8 7

6 5 4

3 2

1

x 1

2

3

4

5

6

7

Buffer solution (b) i.e. 50 cm3 CH 3 COOH and 50 cm3 CH 3 COONa Sr. No. Total Volume of 2M NaOH Resultant added pH 1 0 4.8 2 1 4.8 3 2 4.8 4 3 4.8 5 4 4.9 6 5 4.9 7 6 4.9

8 9 10

7 8 9

4.9 5 5

y 8 7 6 5 4 3 2 1

x 1

2

3

4

5

6

7

Buffer solution (c) i.e. 100 cm3 CH 3 COOH and 100 cm3 CH 3 COONa Sr. No. Total Volume of 2M HCl Resultant added pH 1 0 5 2 1 4.9 3 2 4.9 4 3 4.9 5 4 4.9 6 5 4.9 7 6 4.9 8 7 4.9 9 8 4.9 10 9 4.9 11 10 4.9 12 11 4.9 13 12 4.9 14 13 4.8 15 14 4.8 16 15 4.8 17 16 4.8 18 17 4.8 19 18 4.7 20 19 4.7

y

20

15

10

5

x 2

4

6

8

10

12

14

16

18

Conclusion → Precautions → ► The pH meter should be handled carefully, and special care should be taken to not touch the bulb. ► The pH meter should be washed thoroughly with distilled water before each experiment and when it is introduced into a new solution. ► The measurement of the liquids should be accurate.