ACT science practice test 55

Directions: Each passage is followed by several questions. After reading a passage, choose the best answer to each question and fill in the corresponding oval on your answer document. You may refer to the passages as often as necessary.

You are NOT permitted to use a calculator on this test.


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?

A. 32.569 g of MgSO4 ? nH2O
B. 6.120 g of MgSO4 ? nH2O
C. 29.440 g of MgSO4 ? nH2O
D. 3.129 g of MgSO4 ? nH2O

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

A. He had heated the sample for a sufficient amount of time.
B. The mass of the sample and empty crucible remained at less than 30.00 g for two consecutive heatings.
C. The mass did not change appreciably between the third and fourth heatings.
D. The sample would have started to gain mass if more heating had occurred.

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

A.


B.


C.


D.

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

A. 10% water molecules
B. 25% water molecules
C. 50% water molecules
D. 90% water molecules

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?

A. The calculated percent mass of water would be too high.
B. The calculated percent mass of water would be too low.
C. The calculated percent mass of water would be too high or too low, depending on outside conditions.
D. The calculated percent mass of water would remain unchanged.

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?

A. More water molecules were able to evaporate during the additional 40 minutes.
B. During the additional 40 minutes, the contents cooled to room temperature, which caused the mass to increase.
C. Water molecules were able to reattach to the MgSO4 crystals during the additional 40 minutes.
D. The magnesium sulfate crystals had time to expand during the additional 40 minutes.

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

A. More water would be driven off because the crucible would get to a higher temperature.
B. Drops of water would be found on the lid, but the mass of the sample would not change.
C. Less water would be driven off because the heat could not get to the sample.
D. The mass change of this experiment would be identical to the change in mass recorded in the initial experiment.

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?

A. Both beakers would have identical concentrations of magnesium.
B. Beaker 1 would have a higher concentration of magnesium.
C. Beaker 2 would have a slightly higher concentration of magnesium.
D. Beaker 2 would have a concentration approximately 7 times higher than that of magnesium.