Chem 2113

Test 2

Summer 2000

Questions are worth 25 points each. OMIT ONE QUESTION by clearly writing OMIT in the space provided for your work. If you fail to mark OMIT on a question I will omit the last question of the test. Show your work and circle your answers for full credit.

1. Calculate the pH of the following solutions:

a) 0.025 M perchloric acid

b) 0.010 M chloroacetic acid

c) 0.0010 M ammonium chloride

d) 0.200 M potassium hydrogen phthalate

e) 0.085 M sulfuric acid

2. Consider the titration of 50.0 mL of 0.10 M trimethylamine with 0.10 M HCl. Calculate the pH of the solution at the following points along the titration curve:

a) Before any titrant is added;

b) After 25.0 mL of titrant is added;

c) After 40.0 mL of titrant is added;

d) After 50.0 mL of titrant is added;

e) After 60.0 mL of titrant is added.

3. Consider the titration of 50.0 mL of 0.10 M oxalic acid with 0.20 M NaOH. Calculate the pH of the solution at the following points along the titration curve:

a) Before any titrant is added;

b) After 12.5 mL of titrant is added;

c) After 25.0 mL of titrant is added;

d) After 37.5 mL of titrant is added;

e) After 50.0 mL of titrant is added.

4. A 1.319 g sample containing (NH₄)₂SO₄ (MW = 132.141), NH₄NO₃ (MW = 80.043) and nonreactive substances was diluted to 200.0 mL in a volumetric flask. A 50.00 mL aliquot was made basic with NaOH, and the liberated NH₃ was distilled into 30.00 mL of 0.08412 M HCl. The excess HCl required 10.71 mL of 0.08802 M NaOH. Next, a 25.00 mL aliquot of the original sample was made alkaline after the addition of Devarda's alloy, which reduces the NO₃^- to NH_3. This solution was then distilled into 30.00 mL of the standard acid and back-titrated with 14.16 mL of the base. Calculate the percentage of (NH₄)₂SO₄ and NH₄NO₃ in the original sample.

5. Calculate the fractions for malic acid at a pH of 5.00

A sample possibly containing NaOH (MW = 39.997), Na₂CO₃ (MW = 105.989), and NaHCO₃ (MW = 84.007) was titrated with 0.1000 M HCl. It required 21.86 mL of the acid to reach the phenolphthalein end point, while it required 54.88 mL to reach the methyl red endpoint. Identify the reactive constituents of the mixture, and calculate the mass of each.

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