EXP. 5 BUFFERS, TITRATION CURVES, AND INDICATORS 4. For each of the following, i

Question

EXP. 5 BUFFERS, TITRATION CURVES, AND INDICATORS 4. For each of the following, identify an appropriate indicator: a) 0.1 M HCI and 0.1 M NaOH (equivalence at pH-7) b) 0.25 M HCI and 0.25 M NH3 (equivalence at pH-5) 0.5 M NaOH and 0.5 M weak acid (equivalence at pH=10) 5. In Part 1 of Exp. 5, a student found that the acetic acid concentration was 0.2309 M. The NaOH was known to be 0.1016 M. In Part 2, the student made up a buffer consisting of 60.00 mL of acetic acid and 68.13 mL of NaOH. a) Write the net ionic equation which occurred on mixing the acid and base b) Calculate the concentrations of the weak acid and its conjugate weak base in the resultant buffer. c) What are the major species in solution? d) Did the student make a 1:1 buffer?

Explanation / Answer

4. Indicator

The best indicator will have its color change closest to the equivalence point of the titration.

(a) Phenolphthalein

(b) Phenolphthalein

(c) Phenolphthalein

5. CH3CO2H/CH3CO2Na (weak acid/conjugate base) buffer

a) net-ionic equation,

CH3CO2H + OH- ---> CH3CO2- + H2O

b) [CH3CO2H] in buffer = (0.2309 M x 60 ml - 0.1016 M x 68.13 ml)128.13 ml = 0.0540 M

[CH3CO2Na] in bufffer = (0.1019 M x 68.13 ml)/128.13 ml = 0.0542 M

c) major species in buffer

CH3COOH, CH3COO-

d) No we did not form a 1:1 buffer solution.

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