Chem 2113 Test 2 Summer 1996

Questions are worth 25 points each. Show your work and circle your answers for full credit. Omit one question by leaving completely blank or by writing OMIT in the space provided for your work. Good Luck.

1. Calculate the pH of the following solutions:

a) 0.0025 M HNO3

b) 0.050 M K2O

c) 0.10 M 1-butylamine

d) 0.025 M H3PO4

e) 2.5 x 10-7 M HCl

2. Consider the titration of 50.0 mL of 0.10 M benzoic acid with 0.10 M sodium hydroxide. Calculate the pH of the solution at the following points along the titration curve:

Before any titrant is added;

After 10.0 mL of titrant is added;

After 30.0 mL of titrant is added;

After 50.0 mL of titrant is added;

After 70.0 mL of titrant is added.

3. Consider the titration of 25.0 mL of 0.10 M ethylene diamine with 0.10 M HCl. Calculate the pH of the solution at the following points along the titration curve:

Before any titrant is added;

After 12.5 mL of titrant is added;

After 25.0 mL of titrant is added;

After 37.5 mL of titrant is added;

After 50.0 mL of titrant is added.

4. Consider the determination of nitrogen content in a sample containing protein. After digestion of the 2.500 g sample containing protein, the liberated ammonia is distilled into 100.0 mL of 0.1084 M HCl. It required 14.75 mL of 0.09561 M NaOH to titrate the excess HCl.

Calculate the mass of nitrogen in the sample

Assuming that the protein is 16.4% nitrogen, calculate the percent protein in the original sample.

5. Consider the dissociation of a 0.10 M solution of the weak acid hydrocyanic acid.

Calculate the pH of the solution assuming the ionic strength is equal to 0.

Calculate the pH of the solution assuming the ionic strength is equal to 0.010

Calculate the pH for the 0.10 M HCN in a solution of 0.050 M NaCN. [HINT - what is the ionic strength of this new solution?]

Calculate the relative percent error associated in the determination of the pH of a 0.10 M solution of HCN when the ionic strength is 0.010 if we assume that activities are equal to concentrations.

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