Problem Set III
Chemistry 3100
Winter 2011
Dr. G. Van Biesen
1. By analogy to a hydrogen ion buffer, a metal ion buffer tends to maintain a particular metal ion
concentration in solution. A mixture of HA and A- maintains [H+] via: Ka = [A-][H+]/[HA].
A mixture of CaY2- and Y4-, serves as a Ca2+ buffer via: 1/K'f = [EDTA][Ca2+]/[CaY2-].
How many grams of Na2EDTA·2H2O (FM = 372.23) should be mixed with 1.95 g of
Ca(NO3)·2H2O (FM = 200.12) in a 500 mL volumetric flask to give a buffer with pCa2+ =
9.00 at pH = 9.00?
(This is problem 11-4 in Harris 8th Ed.)
2. Calculate pCu2+ at each of the following points in the titration of 50.00 mL of 0.00100 M Cu2+
with 0.00100 M EDTA at pH 11.0 in a solution whose concentration of NH3 is somehow fixed
at 1.00 M.
Kf for CuY2- is 1018.78;Y for the complexation of Cu2+ with 1 = 9.8x103; 2
= 2.1x107; 3 = 1.15x1010; 4 = 1.07x1012
a) 0 mL; b) 1.00 mL; c) 45.00 mL; d) 50.00 mL; e) 55.00 mL
(This is problem 11-16 in Harris 8th Ed.)
3. A 100 mL aliquot of city drinking water was treated with a small amount of an
ammonia/ammonium chloride buffer to adjust the pH to 10. After the addition of Calmagite,
the solution required 21.46 mL of 5.140 x 10-3 M EDTA for titration. Calculate the water
hardness in mg/L CaCO3.
(This is example 9-6 in Hargis (1988))