The amount of calcium present in milk can be determined by adding oxalate to a sample and measuring the mass of calcium oxalate precipitated. What is the mass percent of calcium if 0.429 g of calcium oxalate forms in a 125-g sample of milk when excess aqueous sodium oxalate is added?
Na2C2O4(aq) + Ca2+(aq) --> CaC2O4(s) + 2Na+(aq)
First, we need to calculate the moles of CaC2O4 formed in the reaction:
0.429 g CaC2O4 x (1 mol CaC2O4 / 128.1 g CaC2O4) = 0.00335 mol CaC2O4
According to the stoichiometry of the reaction, 1 mole of Ca2+ reacts to form 1 mole of CaC2O4. Therefore, 0.00335 mol Ca2+ was present in the 125 g sample of milk.
0.00335 mol Ca x (40.08 g Ca / 1 mol Ca) = 0.134 g Ca
Now, we can calculate the mass percent of calcium in milk:
(0.134 g Ca / 125 g sample) x 100% = 0.107%
Therefore, the mass percent of calcium in the milk sample is 0.107%.