A 0.41 g sample of limestone was dissolved in HCl and all the calcium in the limestone was converted to Ca2+(aq). The solution was treated with excess ammonium oxalate, (NH4)2C2O4(aq), and it was found that 3.0 g of calcium oxalate, CaC2O4(s) was precipitated. Calculate the percentage of calcium in the limestone. (Please give your answer to 4 significant figures)

Answer 1

If 0.41 are the only two digits you have for the sample you don't have enough digits to give an answer to 4 s.f.
Convert 3.0 g CaC2O4 to to grams Ca. That will be 3.0 g CaC2O4 x (atomic mass Ca/molar mass CaC2O4). Then
%Ca = (g Ca/mass sample)*100 = ?
This is a standard procedure for Ca^2+. In limestone one usually must get rid of the R2O3 (Fe3+ and Al^3+) and some other things before pptng CaCO4.