An impure sample of calcium sulphate (CaSO 4 .2H 2 O) was analyzed by gravimetri

Question

An impure sample of calcium sulphate (CaSO4.2H2O) was analyzed by gravimetric analysis of calcium. A (5.090x10^-1) gram sample was dissolved in water and the calcium was precipitated, filtered  and   weighed   as   Ca3(PO4)2. (310.8 g/mole)   A (2.01000x10^-1) gram precipitate was obtained. Calculate the percent calcium (40.08 g/mole) in the sample. [Answer in 4 significant figure]

Note: The system assumes that the values entered are reduced to the highest power possible; e.g. 25.5 = 2.55 x 10^1)

Note: Your answer is assumed to be reduced to the highest power possible

Explanation / Answer

Weight of CaSO4.2H2O = 5.090x10-1 g

Weight of  Ca3(PO4)2 = 2.01000x10-1 g

Molecular weight of  Ca3(PO4)2 = 310.8 g/mole

Therefore 310.8 g/mole contains 120.0 g Calcium

So grams of Ca in 2.01000x10-1 g = 120.0 x 2.01000x10-1 / 310.8 = 0.07760 g = 7.760 x10-2 g

% of Ca in the sample = 7.760 x10-2 g / 5.090x10-1 g = 15.24 % = 1.524 x101 g

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