Finely ground mineral (0.632 4 g) was dissolved in 25 mL of boiling 4 M HCl and diluted with 175 mL H2O containing two drops of methyl red indicator. The solution was heated to 100C, and 50 mL of warm solution containing 2.0 g (NH4)2C2O4 were slowly added to precipitate CaC2O4. Then 6 M NH3 was added until the indicator changed from red to yellow, showing that the liquid was neutral or slightly basic. After slow cooling for 1 h, the liquid was decanted and the solid transferred to a filter crucible and washed with cold 0.1 wt% (NH4)2C2O4 solution five times until no Cl- was detected in the filtrate upon addition of AgNO3 solution. The crucible was dried at 105°C for 1 h and then at 500 ± 25°C in a furnace for 2 h.


The mass of the empty crucible was 18.231 1 g, and the mass of the crucible with CaCO3(s) was 18.546 7 g.
(a) Find the wt% Ca in the mineral.
(b) Why is the unknown solution heated to boiling and the precipitant solution, (NH4)2C2O4, also heated before slowly mixing the two?
(c) What is the purpose of washing the precipitate with 0.1 wt% (NH4)2C2O4?
(d) What is the purpose of testing the filtrate with AgNO3 solution?