Question

calcium in powdered milk is determined by ashing a 1.50g sample and then titrating the calcium with EDTA soluton, 12.1mL being required. The EDTA was standardized by titrating 10.0mL of a zinc solution prepared by dissolving 0.632g zinc metal in acid and diluting to 1L (10.8mL EDTA required for titration). What is the concentration of calcium in the powdered milk in parts per million?

Answer 1

Solution :-

Moles of zinc = 0.632 g / 65.38 g per mol

                          = 0.00967 mol

Moles of zinc reacted = 0.00967 mol * 0.010 L / 1 L = 9.67*10^-5 mol

Mole ratio of the zinc to EDTA is 1 :1 so the moles of EDTA reacted = 9.67*10^-5 mol

So moles of EDTA reacted with calcium = 9.67*10^-5 mol * 12.1 ml / 10.8 ml = 1.08*10^-4 mol

So the moles of the Calcium present = 1.08*10^-4 mol

Now lets calculate the mass of calcium

Mass of calcium = moles * molar mass

                             = 1.08*10^-4 mol * 40.078 g per mol

                            = 0.00433 g

Mass of sample powder = 1.50 g *1 kg / 1000 g = 0.00150 kg

1 ppm = 0.001 g per kg

So

0.00433 g / 0.00150 kg = 2.89 ppm

So the concentration of the calcium is 2.89 ppm