Q: Which of the following is the correct formula for calculating the percent by mass of water in a hydrate?

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

Question 1

A student heated a hydrate sample in a crucible. The mass of the empty crucible was $17.95\ \mathrm{g}$, the mass of the crucible and hydrate before heating was $27.16\ \mathrm{g}$, and the mass after heating (crucible and anhydrous salt) was $26.07\ \mathrm{g}$. What is the mass of water lost during heating?

Accepted Answer

$1.98\ \mathrm{g}$

Question 2

Given the following data: mass of crucible $= 17.95\ \mathrm{g}$, mass of crucible and hydrate $= 27.16\ \mathrm{g}$, mass of crucible and anhydrous salt $= 26.07\ \mathrm{g}$. What is the mass of the anhydrous salt?

Accepted Answer

$8.32\ \mathrm{g}$

Question 3

If the mass of the hydrate is $10.21\ \mathrm{g}$ and the mass of the anhydrous salt is $8.32\ \mathrm{g}$, what is the percent by mass of water in the hydrate?

Accepted Answer

$19.5\%$

Question 4

A hydrate sample loses $1.98\ \mathrm{g}$ of water upon heating. If the molar mass of water is $18.02\ \mathrm{g/mol}$, how many moles of water were lost?

Accepted Answer

$0.110\ \mathrm{mol}$

Question 5

If the mass of anhydrous salt is $8.32\ \mathrm{g}$ and its formula is $\mathrm{CuSO_4}$ (molar mass $159.61\ \mathrm{g/mol}$), how many moles of anhydrous salt are present?

Accepted Answer

$0.0521\ \mathrm{mol}$

Question 6

Using your answers from previous questions, what is the experimental value of $x$ in the hydrate $\mathrm{CuSO_4} \cdot x\mathrm{H_2O}$? (Use moles of water $= 0.110$ and moles of $\mathrm{CuSO_4} = 0.0521$)

Accepted Answer

$2.11$

Question 7

A student determines the mass of water lost from a hydrate to be $1.98\ \mathrm{g}$, but the theoretical value is $2.25\ \mathrm{g}$. What is the percent error in the student's measurement?

Accepted Answer

$12.0\%$

Question 8

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

Accepted Answer

To ensure all water has been removed from the hydrate

Question 9

If a student does not allow the crucible to cool before weighing, what error is most likely to occur?

Accepted Answer

The measured mass will be higher than the true value due to hot air rising

Question 10

A hydrate of $\mathrm{CuSO_4}$ is heated and the following data is collected: mass of hydrate $= 10.21\ \mathrm{g}$, mass of anhydrous salt $= 8.32\ \mathrm{g}$. What is the empirical formula of the hydrate if the molar mass of $\mathrm{CuSO_4}$ is $159.61\ \mathrm{g/mol}$ and $\mathrm{H_2O}$ is $18.02\ \mathrm{g/mol}$?

Accepted Answer

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

Question 11

Which of the following best explains why hydrates lose mass when heated?

Accepted Answer

Water molecules are released as vapor

Question 12

A student accidentally spills some hydrate before weighing the anhydrous salt. How will this affect the calculated percent water in the hydrate?

Accepted Answer

The percent water will be calculated as higher than the true value

Question 13

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

Accepted Answer

The calculated number of waters of hydration will be too high

Question 14

Which of the following is the correct formula for calculating the percent by mass of water in a hydrate?

Accepted Answer

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

Question 15

A hydrate sample is found to contain $0.110\ \mathrm{mol}$ of water and $0.0521\ \mathrm{mol}$ of anhydrous salt. What is the simplest whole number ratio of water to salt?

Accepted Answer

2:1

Question 16

Which of the following best describes the purpose of using a crucible in the hydrate experiment?

Accepted Answer

To provide a heat-resistant container for heating the sample

Question 17

If a hydrate is overheated and decomposes, how will this affect the calculated percent water?

Accepted Answer

The percent water will be calculated as lower than the true value

Question 18

A student calculates the mass of water lost as $1.98\ \mathrm{g}$ and the mass of hydrate as $10.21\ \mathrm{g}$. What is the percent by mass of water in the hydrate?

Accepted Answer

$19.4\%$

Question 19

Which of the following is a common source of error in the determination of the formula of a hydrate?

Accepted Answer

Not heating the sample to constant mass

Analysis of Hydrated Salt: Determining Water Content by Mass Loss

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