Q: Given the following data: mass of hydrate $$= 2.649\ \mathrm{g}$$, mass of water lost $$= 0.903\ \mathrm{g}$$, what is the experimental percent by mass of water in the hydrate?

$$\displaystyle \frac{0.903}{2.649} \times 100 = 39.1\%$$

Q: The theoretical percent by mass of water in $$\mathrm{$$CuSO_4$$ \cdot 5H_2O} is $$closest to:

$$\displaystyle \frac{5 \times 18.02}{159.61 + 5 \times 18.02} \times 100 = 36.1\%$$

Q: If a student experimentally determines the percent by mass of water in $$\mathrm{$$CuSO_4$$ \cdot 5H_2O} to be$$ $$39.1\%$$, what is the percent error compared to the theoretical value of $$36.1\%$$?

$$\displaystyle \frac{39.1 - 36.1}{36.1} \times 100 = 8.3\%$$

Question 1

A student heats a sample of copper(II) sulfate pentahydrate ($$\mathrm{$$CuSO_4$$ \cdot 5H_2O}) to $$drive off the water of hydration. Which of the following best describes the chemical change that occurs during this process?

Accepted Answer

The water molecules are chemically bonded to the copper(II) sulfate and are released as water vapor, leaving behind anhydrous $$\mathrm{$$CuSO_4$$}.$$

Question 2

A student measures the mass of a crucible and lid as $$37.808\ \mathrm{g}$$, the mass of crucible, lid, and hydrate as $$40.457\ \mathrm{g}$$, and the mass of crucible, lid, and anhydrous salt after heating as $$39.554\ \mathrm{g}. $$What is the mass of water lost during heating?

Accepted Answer

$$0.903\ \mathrm{g}$$

Question 3

Given the following data: mass of hydrate $$= 2.649\ \mathrm{g}$$, mass of water lost $$= 0.903\ \mathrm{g}$$, what is the experimental percent by mass of water in the hydrate?

Accepted Answer

$$\displaystyle \frac{0.903}{2.649} 	imes 100 = 39.1\%$$

Question 4

The theoretical percent by mass of water in $$\mathrm{$$CuSO_4$$ \cdot 5H_2O} is $$closest to:

Accepted Answer

$$\displaystyle \frac{5 	imes 18.02}{159.61 + 5 	imes 18.02} 	imes 100 = 36.1\%$$

Question 5

If a student experimentally determines the percent by mass of water in $$\mathrm{$$CuSO_4$$ \cdot 5H_2O} to be$$ $$39.1\%$$, what is the percent error compared to the theoretical value of $$36.1\%$$?

Accepted Answer

$$\displaystyle \frac{39.1 - 36.1}{36.1} 	imes 100 = 8.3\%$$

Question 6

Which of the following is a likely source of error that would cause the experimentally determined percent by mass of water in a hydrate to be higher than the theoretical value?

Accepted Answer

The sample absorbed moisture from the air after heating.

Question 7

A student is asked to determine the value of $$x in$$ $$\mathrm{$$CuSO_4$$ \cdot xH_2O}$$ using the following data: moles of water lost $$= 0.0501\ \mathrm{mol}$$, moles of anhydrous $$\mathrm{$$CuSO_4$$}$$ remaining $$= 0.0109\ \mathrm{mol}. $$What is the value of $$x$$?

Accepted Answer

$$x = \displaystyle \frac{0.0501}{0.0109} = 4.6 \approx 5$$

Question 8

Why is it important to heat the hydrate sample to constant mass during the experiment?

Accepted Answer

To ensure all water of hydration has been removed and only anhydrous salt remains.

Question 9

A student finds that after heating, the mass of the sample continues to decrease with each subsequent heating. What is the most likely explanation?

Accepted Answer

The sample is decomposing after all water has been removed.

Question 10

Which of the following best describes the role of a laboratory balance in quantitative analysis of hydrates?

Accepted Answer

It is used to accurately measure the mass of samples before and after heating to determine the amount of water lost.

Question 11

If a student fails to cool the crucible to room temperature before weighing, what error is most likely to occur?

Accepted Answer

The measured mass will be lower than the true mass due to convection currents.

Question 12

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

Accepted Answer

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

Question 13

A student is analyzing $$2.00\ \mathrm{g} of$$ $$\mathrm{$$NiSO_4$$ \cdot 6H_2O}. $$How many grams of water are present in the sample? (Molar mass of $$\mathrm{$$NiSO_4$$ \cdot 6H_2O} = 262.84\ \mathrm{g/mol}$$, $$6 	imes 18.02 = 108.12\ \mathrm{g}$$ water per mole)

Accepted Answer

$$\displaystyle 2.00 	imes \frac{108.12}{262.84} = 0.823\ \mathrm{g}$$

Question 14

Which of the following best explains why it is necessary to use a lid on the crucible during the initial heating of a hydrate?

Accepted Answer

To minimize the loss of sample due to spattering while allowing water vapor to escape.

Question 15

A student calculates the moles of water lost as $$0.0501\ \mathrm{mol}$$ and the moles of anhydrous $$\mathrm{$$CuSO_4$$} as$$ $$0.0109\ \mathrm{mol}. $$What is the empirical formula of the hydrate?

Accepted Answer

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

Question 16

Which of the following is NOT a proper technique when using a laboratory balance for quantitative analysis?

Accepted Answer

Weighing chemicals directly on the balance pan.

Question 17

If a hydrate sample is not heated long enough, which of the following errors is most likely?

Accepted Answer

The calculated percent by mass of water will be higher than the true value.

Question 18

A student is asked to calculate the percent by mass of water in $$\mathrm{$$MgSO_4$$ \cdot 7H_2O} ($$molar mass $$= 246.54\ \mathrm{g/mol}$$, $$7 	imes 18.02 = 126.14\ \mathrm{g}$$ water per mole). What is the correct calculation?

Accepted Answer

$$\displaystyle \frac{126.14}{246.54} 	imes 100 = 51.2\%$$

Quantitative Analysis of Hydrates: Copper(II) Sulfate and Related Compounds

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