# Solubility Equilibria (Scott)

## Solubility and pH

Oxalic acid, H2C2O4, is a diprotic weak acid (pKa1 = 1.252, pKa2 = 4.266) found in the leaves of the rhubarb plant. Although the stalks of rhubarb are edible, the leaves are quite toxic. For many years this toxicity was believed to be due to oxalic acid’s acidity. More recent work, however, suggests that its toxicity is due to oxalate’s ability to form the insoluble calcium salt, CaC2O4 (Ksp = 1.3×10–8), which crystallizes in the renal tubes and leads to kidney failure. The pH of urine is around 5, but calculating the molar solubility of CaC2O4 at this pH is not easy because at this pH oxalate is present as both HC2O4 and C2O42–; instead, establish some limits by calculating its molar solubility at buffered pH levels of 7.00 and 3.00. For each pH, write the reaction that governs the solubility of CaC2O4, determine the reaction’s equilibrium constant, and then solve for molar solubility.

## Solubility and Complexation Chemistry

One possible treatment for oxalic acid poisoning is to give the patient an oral dose of EDTA, which we abbreviate as Y4–. Because EDTA forms a strong complex with Ca2+ (K1 = 4.90×1010) it may be capable of dissolving CaC2O4 crystals. As a test of the efficacy of this method, 1.00 g of CaC2O4 is placed in a beaker with exactly 100 mL of 0.050 M Y4–. What percentage of the CaC2O4 dissolves and what is the equilibrium concentration of CaY2–? You may assume that the pH is greater than 7.

## Activity and Ionic Strength Workout

1. Calculate the ionic strengths (in M) of solutions with the following compositions:
1. 0.050 M Ca(NO3)2 + 0.098 M Mg(NO3)2
2. 0.25 M HCl + 0.15 M NaCl
3. 0.20 M (NH4)2CrO4
4. 2.65 x 10-2 M LaCl3 + 3.56 x 10-2 M K2SO4
5. 0.29 M NaOH

1. Using activities, calculate the solubility of
1. Ag2SO4 (silver sulfate), Ksp = 1.5 x 10-5, in deionized H2O
2. Ag2SO4 (silver sulfate), Ksp = 1.5 x 10-5, 0.025 M KNO3. Since silver sulfate is sparingly soluble, ignore the contribution of Ag+ and SO42- to the ionic strength.
3. Zn(OH)2, Ksp = 3.0 x 10-16, in 0.020 M K2SO4. 