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Science · Drill 34

Science practice 34

8 questions ~9 min recommended
⏱ 00:00
Score
—

Mass of empty crucible

Mass of crucible + MgSO4 • nH2O sample

Mass of crucible and sample after first heating

Mass of crucible and sample after second heating

Mass of crucible and sample after third heating

Mass of crucible and sample after fourth heating

26.449 g

32.569 g

32.558 g

31.943 g

29.439 g

29.440 g

Mass of empty crucible

Mass of crucible + MgSO4 • nH2O sample

Mass of crucible and sample after first heating

Mass of crucible and sample after second heating

Mass of crucible and sample after third heating

Mass of crucible and sample after fourth heating

Mass of crucible and sample 40 minutes later

26.449 g

32.569 g

32.558 g

31.943 g

29.439 g

29.440 g

32.139 g

12345678

A chemistry student was given the task of determining what percentage of hydrated magnesium sulfate sample was water. Many chemical compounds exist in nature as hydrates instead of in a dry (or anhydrous) state. A sample of hydrated magnesium sulfate (MgSO4 · nH2O) is a solid white powder that contains a certain number (n) of water molecules bonded to each magnesium sulfate crystal. One accepted method of isolating magnesium sulfate from the water molecules is by heating the sample in an open porcelain crucible. Magnesium sulfate has a very high boiling temperature, but water molecules can easily be turned into gas. When sufficiently heated, the water molecules are driven away from the container, leaving only anhydrous magnesium sulfate (MgSO4).

The student cautiously heated the sample over a Bunsen burner. He then removed the crucible, allowed it to cool, and measured the mass. He returned the crucible to the burner and heated it again, allowed it to cool, and measured the mass again. The student repeated this procedure until he was certain all of the water had been removed. His data are found in Table 10.2.

TABLE 10.2

TABLE 10.3

1. What mass of hydrated magnesium sulfate sample did the student place in the crucible at the start of the experiment?

2. How was the student certain that all of the water had been removed from the sample after the fourth heating?

3. Which equation is the appropriate way to determine the percent mass of water in a sample of hydrated magnesium sulfate?

4. From the data in Table 10.2, which is closest to the percent mass of the sample that is made up of water?

5. How would the calculated percent mass of water in the sample be affected if some of the sample splattered out of the crucible while it was being heated?

6. The student left the crucible and the sample on the balance after the fourth heating. Forty minutes later, he came back and noticed the mass of the sample had changed again, as shown in Table 10.3. What is the best conclusion the student can reach about the additional change in mass?

7. The student repeated the procedure but placed a lid on the crucible during the entire experiment. Which statement best describes the expected results?

8. The student did a follow-up experiment by completely dissolving 50 g of anhydrous MgSO4 in 100 mL of distilled water in Beaker 1 and 50 g of MgSO4? 7H2O hydrate in 100 mL of distilled water in Beaker 2. How would the concentration of magnesium (Mg) in each beaker compare?

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