Challenge Problem: Stones form in the urinary tract when certain chemicals become too concentrated in urine. By far the most common kidney stones are those formed from calcium and oxalate. Magnesium is known to inhibit the formation of kidney stones.
(a) The solubility of calcium oxalate (CaC2O4) in urine is 9 × 10-5 M. What is the solubility product, Ksp, of CaC2O4 in urine?
(b) The solubility of magnesium oxalate (MgC2O4) in urine is 0.0093 M. What is the solubility product, Ksp, of MgC2O4 in urine?
(c) The concentration of calcium in urine is approximately 5 mM. What is the maximum concentration of oxalate that can be tolerated and not precipitate CaC2O4?
(d) The pH of Subject A’s urine was 5.9. What fraction of total oxalate, cT,is present as oxalate ion, C2O42-, at pH 5.9? The Ka values for oxalic acid in urine are the same as in water. Hint: Find the ratio [C2O42-]/cT at pH 5.9.
(e) If the total oxalate concentration in Subject A’s urine was 15.0 mM, should a calcium oxalate precipitate form?
(f) In actuality, Subject A does not show the presence of calcium oxalate crystals in urine. Give a plausible reason for this observation.
(g) Why would magnesium inhibit the formation of CaC2O4 crystals?
(h) Why are patients with CaC2O4 kidney stones often advised to drink large amounts of water?
(i) The calcium and magnesium in a urine sample were precipitated as oxalates. A mixed precipitate of CaC2O4 and MgC2O4 resulted and was analyzed by a thermogravimetric procedure. The precipitate mixture was heated to form CaCO3 and MgO. This second mixture weighed 0.0433 g. After ignition to form CaO and MgO, the resulting solid weighed 0.0285 g. What was the mass of Ca in the original sample?
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