Chem 2113 Test 2 Fall 1997

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 HNO₃

0.0025 M HCOOH (Formic acid)

0.050 M C₆H₅NH₂ (Aniline)

0.075 M Na₂C₆H₄(COO)₂ (Sodium phthalate)

0.10 M KClO₄

2. Consider the titration of 50.0 mL of 0.10 M benzoic 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 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 25.0 mL of 0.20 M sodium carbonate with 0.20 M HCl. 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 popular brand of dog food is advertised as having the 'most protein per serving.' Kjeldahl analysis of a 3.29 g sample of the dog food was performed, with the products of the digestion of the dog food being distilled into 100.0 mL of 0.1024 M HCl. The excess HCl was back titrated with 12.08 mL of 0.1007 M NaOH. Given the gravimetric factor for the protein is 6.25 (16.0% nitrogen in the protein), calculate the percent protein in the dog food.

5. Consider a mixture that may contain NaOH (MW = 39.997),Na₂CO₃ (MW = 105.989), and NaHCO₃ (MW = 84.007) along with inert material. A 4.500 g sample of this mixture was dissolved in water and diluted to 200.0 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 17.43 mL of 0.1024 M HCl. The second aliquot, which was titrated to a bromocresol green endpoint, required 49.21 mL of 0.1024 M HCl titrant.

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

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