Answer to Question #338430 in Chemistry for Asha

Question

2. A solution containing 0.402 49 g of CoCl2 ∙ xH2O (a solid with an unknown number of waters of hydration) was exhaustively electrolyzed to deposit 0.09937 g of metallic cobalt on a platinum cathode by the reaction Co2+ + 2e-  Co(s). Calculate the number of moles of water per mole of cobalt in the reagent.

Answer 1

Solution:

%Сo in CoCl₂ is:

%Сo in CoCl₂ = (Molar mass of Co / Molar mass of CoCl₂) = (58.933 / 129.839) = 0.4539 or 45.39%

Therefore, the mass of CoCl₂ in CoCl₂∙xH₂O is:

Mass of CoCl₂ = (Mass of Co / %Сo in CoCl₂) = (0.09937 g / 0.4539) = 0.21893 g

Mass of CoCl₂∙xH₂O = 0.40249 g

Therefore,

Mass of H₂O = Mass of CoCl₂∙xH₂O − Mass of CoCl₂ = 0.40249 g − 0.21893 g = 0.18356 g

Moles of CoCl₂ = (0.21893 g CoCl₂) × (1 mol CoCl₂ / 129.839 g CoCl₂) = 0.001686 mol CoCl₂

Moles of H₂O = (0.18356 g H₂O) × (1 mol H₂O / 18.0153 H₂O) = 0.010189 mol H₂O

Divide the moles by the smallest of the results:

CoCl₂ : 0.001686 / 0.001686 = 1.00

H₂O : 0.010189 / 0.001686 = 6.04

Thus, the formula of CoCl₂∙xH₂O is CoCl₂∙6H₂O (6 moles of water per mole of cobalt)

Answer: 6 moles of water per mole of cobalt in the reagent