Chemistry GIA 10-4
Unit: Acids and Bases Reading: Chapter 19
Working Mode: Pairs. Solve the problems on a separate sheet of notebook paper. Each student must submit his own work. Include the name of your partner.
Learning Objective: Calculate the Ka for a weak acid given appropriate information. Apply understanding of the Ka expression to determine the pH of buffer solutions.
For Problems 1 & 2 be sure to write out the chemical equation for the dissociation of the acid in water, the ICE chart and fill in the chart.
Problem 1: The weak acid, H2CO3, with a concentration of 0.0100 M produces a pH of 4.17, when only one H+ dissociates from some molecules. Determine the Ka of this acid as a monoprotic acid.
Problem 2: Hypochlorous acid, HClO, has a Ka = 3.0 x 10-8. In a 2.5 x 10-2 M HClO solution, the degree of dissociation is so small that the equilibrium concentration of HClO does not noticeably change. What is the equilibrium [ClO-] and the pH of this solution?
Problem 3: A good, balanced buffer has equal concentrations of a weak acid and its conjugate base. Consider a carbonic acid/bicarbonate ion buffer.
(a) Using the Ka you calculated in Problem 1, determine the [H3O+], if the buffer solution contains [H2CO3] and [HCO3-] = 0.0100 M at equilibrium.
(b) What would the pH be?
(c) If the [H2CO3] and [HCO3-] were different than 0.0100 M but still equal to each other, how would that affect the [H3O+]? Explain.
Problem 4: Write the chemical equation for the neutralizing reaction that would occur if a strong acid were added the carbonic acid/bicarbonate ion buffer system.
Problem 5: Write the chemical equation for the neutralizing reaction that would occur if a strong base were added the carbonic acid/bicarbonate ion buffer system.
