1. An uknown sample weighed 2.129g. It contains CoCl 2 * H 2 O and NaCl and was

Question

1. An uknown sample weighed 2.129g. It contains CoCl2 * H2O and NaCl and was heated to drive off all the water. The solid residue that remained weighed 1.712g. Calculate the percentage of CoCl2 * 6H2o in the unknown sample.

3. How can you tell if a compound that gives off water when heated is a true hydrate?

5. Are all hygroscopic substances deliquescent? Why?

6. A drying agent, called Drierite, contains anhydrous CaSO4, a white compound. It is hygroscopic and picks up water. It serves as a drying agent. However, when it gets too wet, it no longer serves as a drying agent. Therefore, some anhydrous cobalt(II) chloride is mixed with the calcium sulfate. When the calcium sulfate is dry so that it can function as a drying agent, Drierite is blue. However, Drierite turns violet or red when it can no longer serve as an useful drying agent. Suggest an explanation of these color changes and explain why we can tell if Drierite in a bottle can be useful as a drying agent or not by looking at its color.

9. What practical use can be made of hygroscopic compounds?

Explanation / Answer

1) The amount of mass lost = 2.129-1.712=0.417g
The weight of water molecule = 18 g / mole

The weight of 6 moles of water = 6X18 = 108 grams

Weight lost = 0.417 grams

So moles of H2O = 0.417 / 18 = 0.0231 moles

So moles of CoCl2 = 0.0231 / 6 = 0.00385 moles

each mole weigh = 129.8 g / mole

weight of CoCl2 = 0.00385 X 129.8 grams = 0.499 grams

Mass of CoCl2x6H2O= 0.499+.417= 0.916 grams
Now percent of compound in sample: 0.916 X 100 /2.129= 43.02 %
3) If we will heat the hydrate there will be some decrease in weight of salt. If we add water to it than there will be again an increase in weight of sample.This process of reversibility can be used to distinguish true hydrates from other compounds that produce water when heated.

5) No all hygroscopic substances are not deliquescent as many hygroscopic substances are not solids.

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