How to find Mg of a substance using molarity

Question

Calcium levels in blood can be determined by adding oxalate ion to precipitate calcium oxalate, CaC2O4, followed by dissolving the precipitate in aqueous acid and titrating the resulting oxalic acid (H2C2O4) with KMnO4:5H2C2O4(aq)+2MnO−4(aq)+6H+(aq)→10CO2(g)+2Mn2+(aq)+8H2O(l).

How many milligrams of Ca2+ are present in 11.0 mL of blood if 23.78 mL of 9.02×10⁻⁴ M KMnO4 solution is needed for the titration?

Answer 1

Since the moles of Ca⁺²=C₂O₄^-2= H₂C₂O_4, the moles of Ca⁺² can be found by using the balanced equation to convert moles of KMNO₄ (which is equal to MnO₄) to moles of H₂C₂O₄ using coefficients in the balanced equation.

find moles of H₂C₂O₄=(0.02378 L)(9.02 x 10^-4 M)=2.14 x 10^-5 mol

mole H₂C₂O₄= 2.14 x 10^-5 mol x (2 mol MnO₄^-/5 mol H₂C₂O₄)=8.58 x 10^-6 mol...this also equals mol of Ca⁺²

to find mg of calcium ion, convert mol to grams and the to mg:

8.58 x 10^-6 mol Ca⁺² x (40 g/mol) x (1000mg/1 g)=0.343 mg Ca⁺²