VI. Buffers and Polyprotic Acids
VI-1. Buffers VI-2. Polyprotic Acids VI-3. Problem List VI-4. Buffer Problems VI-5. Polyprotic Acid Problems

Print this list and try working these problems before checking your answers and looking at the hints.

Buffer Problems

• VI-4(1). Adding 10.0 mL of 0.500 M HCl to 100.0 mL of pH=7 water produces a solution with a pH of 1.34 ([H+] = (0.0100 L)*(0.500 M)/(0.110 L) = 0.0455 M). What is the pH of the resulting solution when 10.0 mL of 0.500 M HCl is added to 100.0 mL of the buffer solution in the introductory example? That buffer contained 0.100 moles of both CH3COOH and CH3COO- in 0.100 L of water?
The Ka of acetic acid is 1.8x10-5
• VI-4(2). What is the pH of the resulting buffer solution when 0.200 moles of HCl are added to 0.500 moles of ammonia, if the solution is then diluted to 0.500 L?
The Kb of ammonia is 1.8x10-5 at 25oC.
• VI-4(3). How many moles of NaOH should be added to a solution containing 0.440 moles of formic acid, HCOOH, to prepare a formic acid/formate buffer with a pH of 4.00?
The Ka of formic acid is 1.7x10-4.

Polyprotic Acid Problems

• VI-5(1). What is the pH of the resulting solution when 100.0 mL of 0.400 M HCl is added to 100.0 mL of 0.300 M sodium carbonate?

H2CO3 (aq) H+(aq) + HCO3-(aq)       Ka1 = 4.45x10-7

HCO3-(aq) H+(aq) + CO32-(aq)         Ka2 = 4.7x10-11

• VI-5(2). How many moles of NaOH must be added to 0.600 moles of phosphoric acid to make a buffer containing equal amounts of HPO42- and PO43-?

H3PO4 (aq) H2PO4-(aq) + H+(aq)        Ka1 = 7.11x10-3     pKa1 = 2.15

H2PO4-(aq) HPO42-(aq) + H+(aq)        Ka2 = 6.34x10-8     pKa2 = 7.20

HPO42-(aq) PO43-(aq) + H+(aq)        Ka3 = 4.20x10-13     pKa3 = 12.38

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