Q: Which of the following is the correct balanced equation for the dehydration of copper(II) sulfate pentahydrate?

$$\mathrm{$$CuSO_4$$ \cdot 5H_2O\ (s) \xrightarrow{\text{heat}} $$CuSO_4$$\ (s) + 5H_2O\ (g)}$$

Question 1

A student heats a sample of hydrated copper(II) sulfate ($$\mathrm{$$CuSO_4$$ \cdot xH_2O}) to $$drive off the water of hydration. If the mass of the hydrated salt before heating is 10.21 g and the mass of the anhydrous salt after heating is 8.32 g, what is the percent by mass of water in the hydrate? Use the formula $$\displaystyle \%\ \mathrm{water} = \frac{\mathrm{mass\ of\ water\ lost}}{\mathrm{mass\ of\ hydrate\ used}} 	imes 100.$$

Accepted Answer

Question 2

Which of the following best describes a hydrate?

Accepted Answer

A compound that contains water molecules chemically bound within its crystal structure.

Question 3

When heating a hydrate to determine its percent water, why is it important to heat, cool, and reweigh the sample multiple times?

Accepted Answer

To ensure all water of hydration has been removed and a constant mass is achieved.

Question 4

A student finds that after heating a hydrate, the mass of the anhydrous salt is 8.32 g and the mass of water lost is 1.89 g. If the molar mass of water is $$18.02\ \mathrm{g/mol}$$, how many moles of water were lost?

Accepted Answer

$$0.105\ \mathrm{mol}$$

Question 5

Given the following data for a hydrate: mass of empty crucible = $$12.75\ \mathrm{g}$$, mass of crucible and hydrate = $$27.16\ \mathrm{g}$$, mass of crucible and anhydrous salt = $$26.07\ \mathrm{g}. $$What is the mass of water lost?

Accepted Answer

$$1.09\ \mathrm{g}$$

Question 6

Which of the following is the correct balanced equation for the dehydration of copper(II) sulfate pentahydrate?

Accepted Answer

$$\mathrm{$$CuSO_4$$ \cdot 5H_2O\ (s) \xrightarrow{	ext{heat}} $$CuSO_4$$\ (s) + 5H_2O\ (g)}$$

Question 7

A student determines that $$0.105$$ moles of water and $$0.052$$ moles of anhydrous salt are present in a sample. What is the value of $$x in $$the formula $$\mathrm{$$CuSO_4$$ \cdot xH_2O}$$ for this hydrate? (Round to the nearest whole number.)

Accepted Answer

$$2$$

Question 8

Why is it important to use a crucible and not a beaker when heating a hydrate to constant mass?

Accepted Answer

A crucible can withstand high temperatures without breaking or reacting with the sample.

Question 9

If a hydrate sample is not heated to constant mass, what is the most likely error in the calculated percent water?

Accepted Answer

The percent water will be too low because not all water was removed.

Question 10

A student is given an unknown hydrate and finds the following: mass of hydrate = $$10.21\ \mathrm{g}$$, mass of anhydrous salt = $$8.32\ \mathrm{g}$$, molar mass of anhydrous salt = $$159.61\ \mathrm{g/mol}. $$Calculate the moles of anhydrous salt present.

Accepted Answer

$$0.052\ \mathrm{mol}$$

Question 11

Which of the following is NOT a common source of error in the determination of percent water in a hydrate?

Accepted Answer

Accurately recording all masses.

Question 12

A hydrate of sodium carbonate has the formula $$\mathrm{Na_2CO_3 \cdot xH_2O}. If a $$student finds $$0.150$$ moles of water and $$0.050$$ moles of anhydrous sodium carbonate, what is the value of $$x$$?

Accepted Answer

$$3$$

Question 13

Which of the following best explains why some hydrates are efflorescent?

Accepted Answer

They spontaneously lose water to the atmosphere because the vapor pressure of the hydrate is greater than that of water vapor in the air.

Question 14

A student calculates the percent water in a hydrate as $$18.5\%. If $$the accepted value is $$36.1\%$$, what is the percent error?

Accepted Answer

$$48.8\%$$

Question 15

Which of the following is the correct formula for barium chloride dihydrate?

Accepted Answer

$$\mathrm{$$BaCl_2$$ \cdot 2H_2O}$$

Question 16

If a hydrate is heated too strongly and some of the salt decomposes, how will this affect the calculated percent water?

Accepted Answer

The percent water will be too high because the mass of anhydrous salt will be too low.

Question 17

A student is analyzing an unknown hydrate. After heating, the mass of the anhydrous salt is higher than expected. Which procedural error most likely occurred?

Accepted Answer

The sample absorbed moisture from the air before weighing.

Question 18

Which of the following is the correct method to determine the number of water molecules per formula unit in a hydrate?

Accepted Answer

Divide the moles of water lost by the moles of anhydrous salt.

Percent of Water in an Unknown Hydrated Salt: Experimental Analysis and Calculations

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