Department of Chemistry and Geosciences

Honors 1084 Lab Spring 2005

Lab Test 5

Name:_________________________________________________ Section:

Answer the following questions to the best of your ability showing all work necessary to obtain the answer. The answer must be given with the correct units and the correct number of significant figures where applicable. Remember to show all your work necessary to obtain the answer. Simply writing the correct answer without showing any work may not receive full credit.

1. (40 points) A standardization was performed by titrating 0.7116 g of KHP (molar mass = 204.2 g/mol) against a sodium hydroxide solution. If it required 28.72 mL of the sodium hydroxide solution to reach the endpoint, what is the molarity of the sodium hydroxide solution?

Calculate the number of moles of KHP:

At the endpoint, the number of moles of NaOH added will be equal to the number of moles of KHP. Calculate the molarity of a NaOH solution that contains 0.003485 moles of NaOH in 28.72 mL of solution:

(4 sig figs in your answer)

2. (20 points) The pH at the half-equivalence point in a titration curve for a weak acid is determined to be 3.85. What is the value of the K_a for this weak acid?

At the half-equivalence point, pH = pK_a = 3.85. Converting to the K_a:

3. (20 points) The K_a for formic acid is equal to 1.77 × 10^-4. What is the value of the pK_a?

Using the definition of pK_a: pK_a = –logK_a = –log(1.77 × 10^-4) = 3.752

4. (20 points) A buffer solution contains roughly equivalent concentrations of:

Either a weak acid and its conjugate base

or

a weak base and its conjugate acid.