Q: Which equation is used to calculate the percent by mass of water in a hydrated salt?

$$\frac{\text{mass of water lost}}{\text{mass of hydrate}} \times 100$$

Q: A student heats a sample of $$\mathrm{$$CuSO_4$$ \cdot xH_2O}$$ and finds the mass of water lost is 1.80 g and the mass of the original hydrate is 10.21 g. What is the percent by mass of water in the sample?

$$\frac{1.80}{10.21} \times 100 = 17.6\%$$

Question 1

Which of the following best describes a hydrated salt?

Accepted Answer

A salt that contains water molecules chemically bonded within its crystal structure.

Question 2

What is the correct formula for copper(II) sulfate pentahydrate?

Accepted Answer

$$\mathrm{$$CuSO_4$$ \cdot 5H_2O}$$

Question 3

During the experiment, why is the crucible heated multiple times when determining the mass of anhydrous salt?

Accepted Answer

To ensure all water of hydration is driven off and a constant mass is achieved.

Question 4

Which equation is used to calculate the percent by mass of water in a hydrated salt?

Accepted Answer

$$\frac{	ext{mass of water lost}}{	ext{mass of hydrate}} 	imes 100$$

Question 5

A student heats a sample of $$\mathrm{$$CuSO_4$$ \cdot xH_2O}$$ and finds the mass of water lost is 1.80 g and the mass of the original hydrate is 10.21 g. What is the percent by mass of water in the sample?

Accepted Answer

$$\frac{1.80}{10.21} 	imes 100 = 17.6\%$$

Question 6

If the molar mass of anhydrous $$\mathrm{$$CuSO_4$$} is 159.61 g/$$mol and a student obtains 8.32 g of anhydrous salt, how many moles of anhydrous salt are present?

Accepted Answer

$$\frac{8.32}{159.61} = 0.0521$$ mol

Question 7

After heating, a student finds the mass of the crucible and anhydrous salt is 8.32 g, and the mass of water lost is 1.89 g. What is the mole ratio of water to anhydrous salt? (Molar mass of water = 18.02 g/mol, molar mass of $$\mathrm{$$CuSO_4$$} = 159.61 g/$$mol)

Accepted Answer

$$\frac{1.89/18.02}{8.32/159.61} = \frac{0.105}{0.0521} = 2.02$$

Question 8

Which of the following is NOT a common hydrated salt listed in the study materials?

Accepted Answer

Sodium chloride monohydrate

Question 9

Why is it important to cool the crucible before weighing it after heating?

Accepted Answer

Hot objects can create air currents that affect the balance reading, leading to inaccurate mass measurements.

Question 10

A student calculates the mole ratio of water to anhydrous salt as 2.02. What is the correct formula for the hydrated salt?

Accepted Answer

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

Question 11

Which process is used to ensure that all water of hydration has been removed from the sample?

Accepted Answer

Heating the sample until a constant mass is achieved after repeated weighings.

Question 12

What is the main reason hydrated salts lose water when heated?

Accepted Answer

The heat provides energy to break the bonds between water molecules and the salt.

Question 13

If a student finds the mass of the crucible and hydrate is 27.16 g, the mass of the empty crucible is 17.95 g, and the mass of the crucible and anhydrous salt after heating is 26.07 g, what is the mass of water lost?

Accepted Answer

$$27.16 - 26.07 = 1.09 g$$

Question 14

Which of the following best explains why the experiment requires the use of a crucible?

Accepted Answer

A crucible can withstand high temperatures needed to drive off water without reacting with the sample.

Question 15

What is the purpose of calculating the mole ratio of water to anhydrous salt in the experiment?

Accepted Answer

To determine the number of water molecules per formula unit in the hydrated salt.

Question 16

Which of the following is a reversible reaction as described in the study materials?

Accepted Answer

$$\mathrm{$$CuSO_4$$ \cdot 5H_2O \xrightarrow{heat} $$CuSO_4$$ + 5H_2O}$$

Question 17

A student finds the mass of water lost is 1.89 g and the moles of anhydrous salt is 0.0521 mol. What is the number of moles of water lost? (Molar mass of water = 18.02 g/mol)

Accepted Answer

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

Question 18

Which of the following is a correct step in the procedure for determining the percent of water in a hydrated salt?

Accepted Answer

Weigh the crucible and hydrate before heating, then weigh the crucible and anhydrous salt after heating.

Percent of Water in an Unknown Hydrated Salt: Principles, Calculations, and Laboratory Procedure

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