40. Solving a chemistry problem

54. To 100 ml of drinking water was added 20 ml of sodium carbonate solution with T (Na2CO3) = 0.0053 g / ml. After boiling and filtering the solution from CaCO3 precipitate, 16.20 ml of HCl solution was used to titrate the remaining Na2CO3 in the filtrate. Calculate the normal concentration of calcium ions in drinking water in mmol/l if 1 ml of HCl solution is equivalent to 1.025 ml of Na2CO3.