The formula for cobalt(II) chloride dihydrate is CoCl2 · 2H2O, which means that each mole of this compound contains 1 mole of CoCl2 and 2 moles of H2O. To find the number of moles of CoCl2 in 56.2 grams of CoCl2 · 2H2O, we need to first find the molar mass of the compound:
Molar mass of CoCl2: 58.933 g/mol
Molar mass of 2H2O: 36.032 g/mol (2 × 18.016 g/mol)
Molar mass of CoCl2 · 2H2O: 58.933 g/mol + 36.032 g/mol = 94.965 g/mol
Now we can find the number of moles of CoCl2 in 56.2 grams of CoCl2 · 2H2O:
56.2 g / 94.965 g/mol = 0.591 moles
Since each mole of CoCl2 · 2H2O contains 1 mole of CoCl2, there are also 0.591 moles of CoCl2 in the sample.
We are told that the hydrate was heated until its weight remained constant, which means that all of the water was driven off and only the anhydrous CoCl2 remained. If the percent yield of the reaction was 100%, we would expect to recover the same amount of CoCl2 that was in the original sample (0.591 moles). However, we are told that the percent yield was only 65.5%, which means that the actual amount of CoCl2 recovered was:
0.655 × 0.591 moles = 0.387 moles
To find the mass of the anhydrous CoCl2 that remained, we can use the molar mass of CoCl2:
Molar mass of CoCl2: 58.933 g/mol
0.387 moles × 58.933 g/mol = 22.8 grams
Therefore, after heating, 22.8 grams of anhydrous cobalt(II) chloride remained.