Chem 2113

Test 2

Fall 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:

a) 0.025 M HNO₂

b) 50.0 mL of 0.050 M HCl mixed with 25.0 mL of 0.025 M NaOH

c) 100.0 mL of 0.075 M HC₂H₃O₂ mixed with 50.0 mL of 0.150 M NaOH

d) 25 mL of 0.10 M HCl mixed with 25 mL of 0.10 M AgCl

e) 0.085 M NaHCO₃

2. Consider the titration of 50.0 mL of 0.10 M formic 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 50.0 mL of 0.050 M sodium phthalate 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 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 0.5834 g sample of a plant food preparation was analyzed for it's N content by the Kjeldahl method; the liberated NH₃ being collected in 50.00 mL of 0.1106 M HCl. The excess acid required an 11.89 mL back titration with 0.1004 M NaOH. Assuming all of the nitrogen present was from the fertilizer urea (H₂NCONH₂), calculate the percent urea in the sample.

5. A 25.00 mL sample of a household cleaning solution was diluted to 250.0 mL in a volumetric flask. A 50.00 mL aliquot of this solution required 40.38 mL of 0.2506M HCl to reach a bromocresol green end point. Calculate the mass of NH₃ per 100 mL of the sample. (Assume all of the alkalinity resulted from the presence of ammonia.)

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