IV. Comparing Buffering Capacity A. pH of Pure Water i. Measure out 30.0 mL DI H

Question

IV. Comparing Buffering Capacity

A. pH of Pure Water

i. Measure out 30.0 mL DI H2O into a beaker.

1. Calculate the expected pH of pure water.                        pH ________

2. Measured pH of pure water.                                                 pH ___3.88______

3. What is the percent error between the calculated and measured value? What are some of the possible sources of this error?

% error __________

B. pH of Water and Strong Acid

i. Measure out 30.0 mL DI H2O and 2.0 mL of 1.0 M HCl into a beaker.

1. Calculate the expected pH of this solution.                      pH _________

2. Measured pH of this solution.                                               pH ___1.64______

3. What is the percent error between the calculated and measured value? What are some of the possible sources of this error?

% error ____

C. pH of Water and Strong Base

i. Measure out 30.0 mL DI H2O and 2.0 mL of 1.0 M NaOH into a beaker.

1. Calculate the expected pH of this solution.                      pH _________

2. Measured pH of this solution. pH __12.03_______

3. What is the percent error between the calculated and measured value? What are some of the possible sources of this error?

% error ____

D. pH of Buffer solution

i. Measure out 50.0 mL 1.0 M CH3COOH and 50.0 mL of 1.0 M NaCH3COO into a beaker.

1. Calculate the total volume. (include units)      Vol __________

2. What is the concentration of acetic acid in this solution after mixing. (include units)

[CH3COOH] _________

3. What is the concentration of acetate ion in this solution after mixing. (include units)

[CH3COO-] ___________

4. Calculate the expected pH of this buffer solution.                        pH __________

5. Measured pH of this buffer solution.                                                 pH __4.56________

6. What is the percent error between the calculated and measured value? What are some of the possible sources of this error?

% error __________

E. pH of Buffer after the addition of the Strong Acid

Measure out 50.0 mL the buffer in part D and add 2.0 mL of 1.0 M HCl into a clean beaker.

1. Calculate the expected pH of this solution.                      pH _________

2. Measured pH of this solution.                                               pH ____5.30_____

3. What is the percent error between the calculated and measured value? What are some of the possible sources of this error?

% error ____

F. pH of Buffer after the addition of the Strong Base

Measure out 50.0 mL the buffer in part D and 2.0 mL of 1.0 M NaOH into a clean beaker.

1. Calculate the expected pH of this solution.                      pH _________

2. Measured pH of this solution. pH _4.57________

3. What is the percent error between the calculated and measured value? What are some of the possible sources of this error?

% error ____

Explanation / Answer

IV. Comparing Buffering Capacity

A. pH of Pure Water

i. Measure out 30.0 mL DI H2O into a beaker.

1. Calculate the expected pH of pure water.                        pH         7

2. Measured pH of pure water.                                                 pH ___3.88______

3. What is the percent error between the calculated and measured value? What are some of the possible sources of this error?

The difference = 7-3.88 = 3.12

% error = 3.12 X 100 / 7 = 44.57

The possible errors are that we have not cleaned the electrode properly

B. pH of Water and Strong Acid

i. Measure out 30.0 mL DI H2O and 2.0 mL of 1.0 M HCl into a beaker.

1. Calculate the expected pH of this solution.                    

Solution:

Molarity of HCl = Initial molarity x initial volume / final volume = 1 X 2 / 32 = 0.0625

pH = -log[H+] = -log[0.0625] = 1.204

2. Measured pH of this solution.                                               pH ___1.64______

3. What is the percent error between the calculated and measured value? What are some of the possible sources of this error?

Solution

Difference = 1.64 – 1.204 = 0.436

% difference = 0.436 X 100 / 1.204 = 36.2%

C. pH of Water and Strong Base

i. Measure out 30.0 mL DI H2O and 2.0 mL of 1.0 M NaOH into a beaker.

1. Calculate the expected pH of this solution.

Solution

Molarity of NaOH after addition of water = Initial molarity of NaOH x volume of NaOH / Total volume

Molarity of NaOH = 2/32 = 0.0625

pOH = -log[OH-] = 1.204

pH = 14-1.204 = 12.796

2. Measured pH of this solution. pH __12.03_______

3. What is the percent error between the calculated and measured value? What are some of the possible sources of this error?

Difference = 12.796 – 12.03 = 0.766

% error = 0.766 X 100 / 12.796 = 5.99%

D. pH of Buffer solution

i. Measure out 50.0 mL 1.0 M CH3COOH and 50.0 mL of 1.0 M NaCH3COO into a beaker.

1. Calculate the total volume. (include units)      Vol = 100mL

2. What is the concentration of acetic acid in this solution after mixing. (include units)

[CH3COOH] = initial concentration X initial volume / final volume

[CH3COOH] = 1 X 50 / 100 = 0.5 M

3. What is the concentration of acetate ion in this solution after mixing. (include units)

[CH3COO-] = initial concentration X initial volume / final volume

[CH3COO-] = 1 X 50 / 100 = 0.5 M

4. Calculate the expected pH of this buffer solution.                       

For buffers

pH = pKa + log [salt/ [acid]

pKa of acetic acid = 4.74

pH = 4.74 + log[0.5*0.5] = 4.74

Measured pH of this buffer solution.                                                 pH __4.56________

6. What is the percent error between the calculated and measured value? What are some of the possible sources of this error?

Difference in values = 4.74-4.56 = 0.18

% error = 0.18 X 100 / 4.74 = 3.797 %

E. pH of Buffer after the addition of the Strong Acid

Measure out 50.0 mL the buffer in part D and add 2.0 mL of 1.0 M HCl into a clean beaker.

The number of moles of HCl added = Molarity X volume = 1 X 2 millimoles = 2mmol

This will react with same mmol of acetate ion to give same mmol of acetic acid

Mmol of acetic acid initially present in buffer = 0.5 X 100 = 50mmol

Mmol of acetate ion present in buffer = 0.5 X 100 = 50mmol

Mmol of acetic acid after addition of HCl = 50+2 = 52mmol

Mmol of acetate ion after addition of HCl = 50-2 = 48mmol

pH = pKa + log [salt] / [acid] = 4.74 + log [48/52] = 4.71

2. Measured pH of this solution.                                               pH ____5.30_____

3. What is the percent error between the calculated and measured value? What are some of the possible sources of this error?

Difference = 5.30-4.71 = 0.59

% difference = 0.59 X 100 / 4.71 = 12.53%

F. pH of Buffer after the addition of the Strong Base

Measure out 50.0 mL the buffer in part D and 2.0 mL of 1.0 M NaOH into a clean beaker.

The number of moles of NaOH added = Molarity X volume = 1 X 2 millimoles = 2mmol

This will react with same mmol of acetic acid to give same mmol of acetate ion

Mmol of acetic acid initially present in buffer = 0.5 X 100 = 50mmol

Mmol of acetate ion present in buffer = 0.5 X 100 = 50mmol

Mmol of acetate ion after addition of NaOH = 50+2 = 52mmol

Mmol of acetic acid after addition of NaOH = 50-2 = 48mmol

pH = pKa + log [salt] / [acid] = 4.74 + log [52/48] = 4.77

2. Measured pH of this solution. pH _4.57________

3. What is the percent error between the calculated and measured value? What are some of the possible sources of this error?

Difference = 4.77-4.57 = 0.2

% difference = 0.2 X 100 / 4.77 = 4.19%

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