Q: If the mass of anhydrous salt is 8.32 g and the mass of water lost is 1.89 g, what is the percent by mass of water in the hydrate?

$$\frac{1.89}{10.21} \times 100 = 18.5\%$$

Q: A hydrate sample is analyzed and found to contain 18.5% water by mass. If a 5.00 g sample is used, how many grams of water are present?

$$5.00 \times 0.185 = 0.925\ \mathrm{g}$$

Question 1

A student heated a hydrate sample in a crucible to remove water. The mass of the empty crucible was 17.95 g, the mass of crucible plus hydrate was 27.16 g, and the mass of crucible plus anhydrous salt after heating was 26.07 g. What is the mass of water lost during heating?

Accepted Answer

$$10.21 - 8.32 = 1.89\ \mathrm{g}$$

Question 2

Given the mass of crucible plus hydrate is 27.16 g and the mass of the empty crucible is 17.95 g, calculate the mass of the hydrate sample used.

Accepted Answer

$$27.16 - 17.95 = 9.21\ \mathrm{g}$$

Question 3

If the mass of anhydrous salt is 8.32 g and the mass of water lost is 1.89 g, what is the percent by mass of water in the hydrate?

Accepted Answer

$$\frac{1.89}{10.21} 	imes 100 = 18.5\%$$

Question 4

A hydrate sample loses water upon heating. Which of the following best describes the chemical process occurring?

Accepted Answer

Decomposition reaction: $$\mathrm{Hydrate} \rightarrow \mathrm{Anhydrous\ salt} + \mathrm{Water}$$

Question 5

If the hydrate is $$\mathrm{$$CuSO_4$$ \cdot xH_2O}$$ and the molar mass of anhydrous $$\mathrm{$$CuSO_4$$} is$$ $$159.6\ \mathrm{g/mol}$$, how many moles of anhydrous salt are present in 8.32 g?

Accepted Answer

$$\frac{8.32}{159.6} = 0.0521\ \mathrm{mol}$$

Question 6

Given the mass of water lost is 1.89 g, calculate the number of moles of water lost. (Molar mass of $$\mathrm{H_2O} is$$ $$18.02\ \mathrm{g/mol})$$

Accepted Answer

$$\frac{1.89}{18.02} = 0.105\ \mathrm{mol}$$

Question 7

Using your calculated moles of anhydrous salt ($$0.0521\ \mathrm{mol}) $$and moles of water ($$0.105\ \mathrm{mol}$$), determine the value of $$x in$$ $$\mathrm{$$CuSO_4$$ \cdot xH_2O}$$ for this hydrate sample.

Accepted Answer

$$x = \frac{0.105}{0.0521} = 2.02$$

Question 8

Why is it important to heat the hydrate sample multiple times until a constant mass is achieved?

Accepted Answer

To ensure all water has been removed and the sample is fully converted to the anhydrous salt.

Question 9

If a student forgets to subtract the mass of the empty crucible when calculating the mass of the hydrate, what error will result?

Accepted Answer

The calculated mass of the hydrate will be much larger than the actual value.

Question 10

A hydrate sample is analyzed and found to contain 18.5% water by mass. If a 5.00 g sample is used, how many grams of water are present?

Accepted Answer

$$5.00 	imes 0.185 = 0.925\ \mathrm{g}$$

Question 11

Which of the following best explains why hydrates are used in medicine and pharmaceuticals?

Accepted Answer

Hydrates can stabilize active ingredients by incorporating water molecules into their crystal structure.

Question 12

A student determines the mass of anhydrous salt to be 8.32 g after heating. If the theoretical yield is 8.50 g, what is the percent yield?

Accepted Answer

$$\frac{8.32}{8.50} 	imes 100 = 97.9\%$$

Question 13

If a hydrate sample is overheated and decomposes, which of the following is a likely consequence?

Accepted Answer

The mass of the anhydrous salt will be lower than expected due to decomposition.

Question 14

Which laboratory technique is most appropriate for determining the water content in a hydrate?

Accepted Answer

Heating to constant mass in a crucible.

Question 15

A hydrate sample is found to have a molar ratio of water to anhydrous salt of 2:1. What is the empirical formula of the hydrate?

Accepted Answer

$$\mathrm{$$CuSO_4$$ \cdot 2H_2O}$$

Question 16

If a student uses a wet crucible (not fully dried before use), what error will this introduce in the calculation of water lost?

Accepted Answer

The calculated mass of water lost will be artificially high.

Question 17

Which of the following is the best reason for using a lid on the crucible during heating?

Accepted Answer

To prevent loss of sample due to splattering while allowing water vapor to escape.

Question 18

A student calculates the percent by mass of water in a hydrate as 18.5%. If the accepted value is 20.0%, what is the percent error?

Accepted Answer

$$\frac{|18.5 - 20.0|}{20.0} 	imes 100 = 7.5\%$$

Hydrates: Experimental Determination of Water Content in Ionic Compounds

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