Question

1. Some compounds that exist as hydrates can be used as indicators. One commonly used indicator is cobalt(II) chloride (CoCl2). Anhydrous cobalt(II) chloride is blue. When it bonds with water molecules, it turns purple to make the hydrate. Write chemical equation for the reaction that occurs. 2. Is this reaction a combination, decomposition, single replacement or double replacement reaction? 3. If 41.46 grams of cobalt(II) chloride hydrate is heated to remove the bonded water and the product weighs 32.46 grams, what is the formula of cobalt(II) chloride hydrate? Based on your calculations, what is the correct name for the hydrate? (Include the correct prefix for the water.) 4. What is the % by mass of water in the hydrate?

Answer 1

1. Equation of reaction: CoCl₂ + 6 H₂O $\rightarrow$ CoCl₂.6H₂O

2. This is a combination reaction.

3. Mass of CoCl₂ = 32.46 g

moles of salt = mass / molar mass = 32.46/129.8 = 0.25

We know that moles of Hydrate salt (CoCl₂.xH₂O) = moles of dry salt = 0.25

mass of hydrate salt / molar mass of hydrate salt = 0.25

41.46 / (129.8 + 18x) = 0.25

Solving,we get x = 36.04 / 18 = 2

Formula : CoCl₂.2H₂O

4. Mass of hydrated salt = 165.8

Mass of water in the hydrated salt = 2 * 18 = 36

Mass % of water = mass of water * 100 / mass of hydrated salt = 36 * 100 / 165.8 = 21.71%