Question

A student wants to determine the calcium content in a vitamin tablet. A 2.567 g tablet was dissolved in 10 mL of nitric acid and then quantitatively transferred to a 100.00 mL flask. From this flask, a 25.00 mL aliquot was taken and (NH₄)₂C₂O₂ added to form CaC₂O₂ as a precipitate. The solid was transferred to a gooch crucible, dried and 0.8973 g of precipitate collected. How much Ca in grams was in the tablet?

                                    Ca⁺² (aq)   + C₂O₂^-2 (aq)   --->   CaC₂O₂ (s)

Answer 1

m = 2.567 g of sample

V = 10 mL nitric acid

m = 0.8973 g of precipitate

MW of CaC2O2  

mol = mass/MW = 0.8973 /96.0982 = 0.009337 mol of CaC2O2

note that 1 mol of CaC2O2 = 1 mol of Ca+2

so

0.009337 mol of Ca+2

MW of Ca = 40.04 g/mol

mass of Ca = mol*MW = 0.009337 * 40.04 = 0.373853 g of Ca were in those 25 mL

NOTE that I will assume that in the Flask, we used 100 mL to dilute it (it does not state it)

so

we have 1/4 of the amount

so

total mass = 4*0.373853 = 1.495412 g or about 1.5 grams of Ca in sample