Question

V. Buffer Solution from Strong Base and Weak Acid Measure out 30.0 mL of 1.0 M CH3COOH and 5.0 mL of 1.0 M NaOH 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 sodium hydroxide in this solution after mixing. (include units) [NaOH] __________

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

_ 5. Measured pH of this buffer solution. pH ___3.77_______

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

Answer 1

[CH3COOH] = 30 mL x 1 M = 30 mmol

[NaOH] = 5 mL x 1 M = 5 mmol

         CH3COOH   + NaOH       ------------------------> CH3COONa + H2O

Initial            30 mmol              5 mmol                                      0

----------------------------------------------------------------------------------------------------------------

after mixing        30-5                    0                                         5 mmol

                         = 25 mmol

Therefore, after mixing

[CH3COOH] = 25 mmol = 0.025 mol

[NaOH] = 0

[CH3COONa] = 5 mmol = 0.005 mol

We know that pKa of acetic acid CH3COOH = 4.75

From Hendeson-Hasselbalch equation,

pH = pKa + log [salt]/[acid]

     = 4.75 + log (0.005 mol/ 0.025 mol)

   = 4.05

pH = 4.05

Therefore,

1) Calculate the total volume. (include units) Vol = 30 ml + 5 ml = 35 ml = 0.035 L

2) What is the concentration of acetic acid in this solution after mixing. (include units) [CH3COOH] = 0.025 mol

   (or) 0.025 mol/ 0.035 L = 0.714 M

3) What is the concentration of sodium hydroxide in this solution after mixing. (include units) [NaOH] = 0

4) Calculate the expected pH of this buffer solution. pH = 4.05

6) What is the percent error between the calculated and measured value ?

                 = (4.05-3.77) / 3.77 x 100

                 = 7.4 %