21. Solving a chemistry problem

6. From a sample of 0.1000 g of limestone, calcium was isolated in the form of oxalate and dissolved in 2N HCl. The volume of the solution was brought to the mark in a 100 ml volumetric flask. Then, 10 ml of the resulting solution were transferred into three 100 ml volumetric flasks; 10 and 20 ml of standard 7.5*10-3 M CaCl2 solution were added to the second and third flasks, respectively. The volumes of the solutions were brought to 100 ml and their photometry was carried out on a flame photometer. Determine the percentage (by mass) content of CaCO3 in limestone, if the microammeter reading for the indicated solutions was 15, 156 and 296 μA, respectively.