# AP CHEMISTRY 2008 SCORING GUIDELINES

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## 2008 SCORING GUIDELINES

### Question 2

Answer the following questions relating to gravimetric analysis.

In the first of two experiments, a student is assigned the task of determining the number of moles of water in one mole of MgCl2⋅ n H2O. The student collects the data shown in the following table.

Mass of empty container 22.347 g

Initial mass of sample and container 25.825 g

Mass of sample and container after first heating 23.982 g

Mass of sample and container after second heating 23.976 g

Mass of sample and container after third heating 23.977 g

(a) Explain why the student can correctly conclude that the hydrate was heated a sufficient number of times in the experiment.

No additional mass was lost during the third heating, indicating that all the water of hydration had been driven off.

One point is earned for the correct explanation.

(b) Use the data above to

(i) calculate the total number of moles of water lost when the sample was heated, and mass of H2O lost = 25.825 − 23.977 = 1.848 g OR 25.825 − 23.976 = 1.849 g 1.848 g H2O × 2 2 1 mol H O 18.02 g H O = 0.1026 mol H2O

One point is earned for calculating the correct number of moles of water.

(ii) determine the formula of the hydrated compound.

mass of anhydrous MgCl2 = 23.977 − 22.347 = 1.630 g 1.630 g MgCl2 × 2 2 1 mol MgCl 95.20 g MgCl = 0.01712 mol MgCl2 2 2 0.1026 mol H O

0.01712 mol MgCl = 5.993 ≈ 6 mol H2O per mol MgCl2 ⇒ formula is MgCl2 x6H2O

One point is earned for calculating the correct number of moles of

anhydrous MgCl2.

One point is earned for writing the correct formula (with supporting

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## 2008 SCORING GUIDELINES

### Question 2 (continued)

(c) A different student heats the hydrate in an uncovered crucible, and some of the solid spatters out of the crucible. This spattering will have what effect on the calculated mass of the water lost by the hydrate? Justify your answer.

The calculated mass (or moles) of water lost by the hydrate will be too large because the mass of the solid that was lost will be assumed to be water when it actually included some MgCl2 as well.

One point is earned for the correct answer with justification.

In the second experiment, a student is given 2.94 g of a mixture containing anhydrous MgCl2 and KNO3 . To determine the percentage by mass of MgCl2 in the mixture, the student uses excess AgNO3(aq)to precipitate the chloride ion as AgCl(s).

(d) Starting with the 2.94 g sample of the mixture dissolved in water, briefly describe the steps necessary to quantitatively determine the mass of the AgCl precipitate.

- Separate the AgCl precipitate (by filtration).

- Wash the precipitate and dry the precipitate completely. - Determine the mass of AgCl by difference.

Two points are earned for all three major steps: filtering the mixture, drying the

precipitate, and determining the mass by difference.

One point is earned for any two steps.

(e) The student determines the mass of the AgCl precipitate to be 5.48 g. On the basis of this information, calculate each of the following.

(i) The number of moles of MgCl2 in the original mixture

5.48 g AgCl × 1 mol AgCl

143.32 g AgCl = 0.0382 mol AgCl

0.0382 mol AgCl × 1 mol Cl

1 mol AgCl ×

2

1 mol MgCl

2 mol Cl = 0.0191 mol MgCl2

One point is earned for calculating the number of

moles of AgCl. One point is earned for conversion to moles of

MgCl2. (ii) The percent by mass of MgCl2 in the original mixture

0.0191 mol MgCl2 × 2 2 95.20 g MgCl 1 mol MgCl = 1.82 g MgCl2 1.82 g MgCl2 2.94 g sample × 100% = 61.9% MgCl2 by mass

One point is earned for calculating the correct percentage.

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## 2008 SCORING COMMENTARY

### Question 2

Overview

This question assessed student knowledge and skills relating to gravimetric analysis, which is included in several of the laboratory experiments recommended in the AP Chemistry Course Description. In parts (a) through (c) students were asked to analyze and interpret a data table. They had to explain how they correctly determined that all the water of hydration had been driven off from a sample of a hydrate; calculate an appropriate formula for the hydrate; and determine the effect of an error in laboratory procedure on the calculation of the mass of water released upon heating. Parts (d) and (e) required students to describe a quantitative laboratory procedure to determine the mass of a precipitate from a mixture and then calculate the number of moles and percent by mass of a component of the mixture.

Sample: 2A Score: 10

This response earned all 10 points: 1 for part (a), 1 for part (b)(i), 2 for part (b)(ii), 1 for part (c), 2 for part (d), 2 for part (e)(i), and 1 for part (e)(ii).

Sample: 2B Score: 8

In part (d) no points were earned because the necessary steps were not described. Sample: 2C

Score: 6

In part (d) no points were earned because the necessary steps were not described. In part (e)(i) 1 point was earned for dividing by the molar mass of AgCl, but the other point was not earned owing to the misapplication of the mole ratio in the calculation. In part (e)(ii) no point was earned. Although the answer brought down from part (e)(i) was multiplied by the correct molar mass, that product was not subsequently divided by 2.94 g, the mass of the original sample.
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