A sample of a crystalline compound, when heated in an open test tube, produced s

Question

A sample of a crystalline compound, when heated in an open test tube, produced several droplets of water on the cool upper region of the tube. The residue dissolved in water forming a yellow-brown solution, while the original compound formed a colorless solution in water. Why should you conclude that the compound is not a true hydrate? What is the meaning of the numeral represented by X in the general formula for a hydrate, salt .XH_2O? Cobalt(II) chloride is commonly obtained from chemical supply houses as a hydrate with the formula CoCl_2 middot 6 H_2O. An analysis showed that 25.0 g of this hydrate contains 11.3 g of water. What is the percent water by weight in this hydrate? Please show your calculations below. Percent water =

Explanation / Answer

2) When the hydrate is heated it tends to loose its water of hydration which appear as droplets in the cooler regions of the test tube. It is mentioned that the original hydrate forms a colorless solution when dissolved in water. Now, if the hydrate was a 'true hydrate' then the residue left in the test tube when once again dissolved in water should also form a colorless solution. Instead, it forms a yellowish brown solution, which indiactes that it is not a true hydrate.

3) General formula of a hydrate is: salt.XH2O

Here, X refers to the water of hydration. i.e the water present in these compounds as part of their crystalline structure.

4) Given hydrate: CoCl2. 6H2O

Mass of hydrate = 25 g

Mass of water = 11.3 g

% H2O = (mass of water/mass of hydrate)*100 = (11.3/25)*100 = 45.2%

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