Chem 2113

Test 2

Summer 1998

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:

0.0025 M HClO₄

0.0025 M benzoic acid

0.050 M NaHC₆H₄(COO)₂ (sodium hydrogen phthalate)

0.075 M Na₃PO₄ (sodium phosphate)

0.10 M NaNO₃

2. Consider the titration of 50.0 mL of 0.10 M methyl ammonia 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 15.0 mL of titrant is added;

c) After 25.0 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 0.050 M malonic acid with 0.10 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) A 50.00 mL sample of a white dinner wine required 21.48 mL of 0.03776 M NaOH to achieve a phenolphthalein endpoint. Express the acidity of the wine in terms of grams of tartaric acid (H₂C₄H₄O₆, 150.09 g/mol) per 100 mL. (Assume that both of the acidic hydrogens are titrated.)

b) The active ingredient in disulfiriam, a drug used for the treatment of chronic alcoholism, is tetraethylthiurram disulfide. This drug has a molecular weight of 296.55, and contains four (4) sulfur atoms per molecule. The sulfur in a 0.4329 g sample of disulfiriam preparation was oxidized to SO₂, which was absorbed in H₂O₂ to give H₂SO₄.The acid was treated with 22.13 mL of 0.03736 M NaOH. Calculate the percentage tetraethylthiurram disulfide in the preparation.

5. Consider a mixture that may contain NaOH, Na₂CO₃, and NaHCO₃ along with inert material. A 1.000 g sample of this mixture was dissolved in water and diluted to 100.0 mL in a volumetric flask. Two 25.00 mL aliquots of this solution were titrated. The first aliquot, which was titrated to a phenolphthalein endpoint required 28.31 mL of 0.1092 M HCl. The second aliquot, which was titrated to a bromocresol green endpoint, required 39.91 mL of 0.1092 M HCl titrant.

a) Which specie(s) are present? Explain.

b) Calculate the percent of each specie(s) present in the original sample.

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