Final answer:
After heating off all the water from a 1.691-gram sample of cobalt(II) chloride hexahydrate, approximately 0.9236 grams of anhydrous cobalt(II) chloride would remain.
Step-by-step explanation:
When heating cobalt(II) chloride hexahydrate to remove the water, you're left with the anhydrous form of cobalt(II) chloride, CoCl₂. The hexahydrate formula, CoCl₂ · 6H₂O, indicates each formula unit includes six water (6H₂O) molecules. To find the mass that remains, calculate the mass of the anhydrous cobalt(II) chloride by subtracting the mass of the water from the total mass of the hydrated compound.
Firstly, find the molar mass of the water in the compound: 6 molecules of water (H₂O, with a molar mass of 18.015 g/mol) gives 6 × 18.015 = 108.09 g/mol water. The molar mass of cobalt(II) chloride hexahydrate (CoCl₂ · 6H₂O) is 237.93 g/mol. Therefore, the mass fraction of the water in the compound is (108.09 g/mol) / (237.93 g/mol) = 0.454 or 45.4%.
To calculate the expected mass remaining after the water has been removed from a 1.691-gram sample, multiply the sample mass by the percentage of the anhydrous compound (100% - 45.4% = 54.6%): 1.691 g × 0.546 = 0.9236 g. Therefore, we expect approximately 0.9236 grams of anhydrous cobalt(II) chloride after heating off all the water.