Question

A solution containing .40249 g of CoCl₂ * xH₂O ( a solid with an unknown number of waters of hydration) was exhaustively electrolyzed to deposit .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

Answer – We are given,

Mass of CoCl₂. xH₂O = 0.40249 g

Mass of Co = 0.09937 g

We need to calculate the moles of Co

Moles of Co = 0.09937 g / 58.933 g.mol^-1

                 = 0.00169 moles of Co

From the given reaction equation

Co²⁺ + 2e^- ----> Co(s)

Moles of Co = moles of Co²⁺

So, moles of Co²⁺ = 0.00169 moles

moles of Co²⁺ = moles of CoCl₂ = 0.00169 moles

mass of CoCl₂ = 0.00169 moles X 129.84 g/mol

                      = 0.21893 g of CoCl₂

So, mass of water in CoCl₂. xH₂O = 0.40249 g - 0.21893 g

                                                   = 0.18356 g of H₂O

Moles of water = 0.18356 g of H₂O / 18.016 g.mol^-1

                       = 0.010189 moles

Number of moles of water per mole of cobalt in the reagent      

   = 0.010189 moles / 0.00169 moles

   = 6