ACT science practice test 79

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.


Radium-226 (226Ra) activity is often elevated in bodies of water that have been augmented with groundwater. Samples were collected over a two-year period in order to study 226Ra activity in Florida Marsh and Cypress wetlands. Figure 1 shows the average 226Ra activity from surface sediment samples collected in 2002 and 2004. Cores were taken from shallow sediment (0 to 4 cm) in the wetlands and analyzed for 226Ra activity. The activity, in disintegration per minute per gram (dpm/g), found in the Marsh and Cypress wetlands is displayed in Figures 2 and 3.

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Figure 1

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Figure 2

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Figure 3

1. According to Figure 2, the average radium-226 activity in Cypress wetlands in 2004 at a depth of 200 cm was closest to which of the following?

A. 0.30 dpm/g
B. 0.70 dpm/g
C. 1.00 dpm/g
D. 2.50 dpm/g

2. Based on the information in Figure 3, if the average radium-226 activity had been measured in Marsh wetlands at the depth of 400 cm in 2002, it most likely would have been closest to which of the following?

F. 2.75 dpm/g
G. 2.50 dpm/g
H. 2.00 dpm/g
J. 1.80 dpm/g

3. Which of the following is the most likely explanation for the difference in the average 226Ra activity between 2002 and 2004?

A. In 2004 the wetlands were augmented with groundwater, which caused an increase in 226Ra activity.
B. In 2002 the wetlands were augmented with groundwater, and in 2004, they were not. This resulted in more 226Ra activity in 2004.
C. The amount of groundwater the wetlands were augmented with was higher in 2002 than in 2004.
D. In 2004 there was more rainfall than in 2002, which created a decrease in 226Ra activity.

4. If the data in Figures 2 and 3 are typical of Cypress and Marsh wetlands, one would most likely make which of the following conclusions about the 226Ra activity in a wetland?

F. 226Ra activity is highest in wetlands in the top 50 cm of the sediment layer.
G. 226Ra activity gets higher as the sediment gets deeper.
H. 226Ra activity is highest in wetlands at around 325 cm below the top of the sediment.
J. Each year, the 226Ra activity in wetlands will become less.

5. According to Figures 2 and 3, the 226Ra activity increased between the depths of 200 and 250 cm for which wetlands?

A. Cypress wetlands in 2002
B. Cypress wetlands in 2004
C. Marsh wetlands in 2002
D. Marsh wetlands in 2004

The balanced chemical equation for the dissociation of acetic acid in water is shown below:

CH3COOH(aq) imageCH3COO- (aq) + H+(aq)

The reaction can proceed in either direction in order to maintain a constant value for the acid dissociation constant. Acetic acid is an acid because it contributes H+ ions to the solution. The acid dissociation constant for the above reaction can be expressed as follows:

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The expression [A] represents the concentration of substance A in moles/liter. One mole is 6.02 · 1023 molecules.

Experiment 1

NaOH is a strong base because it dissociates readily in water and contributes OH ions to the water. A 1 M solution of NaOH, a strong base, is slowly added to a 100 ml, 2 M (M represents molarity, a measure of concentration in image) solution of acetic acid. The NaOH solution contributes OH ions to the mixture. An ion is a charged atom or molecule. The OH ions from the NaOH combine with the H+ ions from the acetic acid to form water as shown below:

H+(aq) + OH (aq) → H2O (l)

Various measurements were taken and the results are shown in Table 1 and graphed in Figure 1. Note: pH = –log [H+]

Table 1

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Figure 1

6. Using the figure and the table, how much NaOH was added in order to increase the pH to 9.3?

F. 0 ml
G. 100 ml
H. 200 ml
J. 300 ml

7. If only 50 ml of NaOH had been added, what approximate value would the pH have been?

A. 1.5
B. 3.5
C. 5.5
D. 7.5

8. Using all the information in the passage, why did the concentration of CH3COOH(aq) continue to decrease while the concentration of CH3COO(aq) remained nearly unchanged?

F. Looking at the Ka, [CH3COOH(aq)] is in the denominator and [CH3COO(aq)] is in the numerator. The system counteracts the decrease in [H+(aq)] by dissociating more molecules of CH3COOH.
G. Looking at the Ka, [CH3COOH(aq)] is in the denominator and [CH3COO(aq)] is in the numerator. The system counteracts the increase in [H+(aq)] by synthesizing more molecules of CH3COOH.
H. The NaOH produces Na+ ions that must be combining with the CH3COOH molecules in such a way that the concentration continues to decrease.
J. Clearly, the data must be inaccurate. Looking at the balanced equation, for every molecule of acetic acid that dissociates, one CH3COO ion must form and the [CH3COO(aq)] should have continued to increase.

9. What would be the effect on the results if 100 ml of 1 M acetic acid was used instead of 100 ml of 2 M?

A. The decreased concentration of acetic acid will decrease acidity and increase the initial pH.
B. The decreased concentration of acetic acid will decrease acidity and decrease the initial pH.
C. The increased concentration of acetic acid will increase acidity and increase the initial pH.
D. The increased concentration of acetic acid will increase acidity and decrease the initial pH.

10. How could you lower the pH back to 2.2?

F. Adding enough NaOH will eventually lower the pH because the cycle will repeat.
G. Adding enough water will dilute the base and lower the pH.
H. Evaporating some of the water will increase the concentration of H+ ions sufficiently to lower the pH.
J. Adding a sufficient amount of acetic acid will add H+ ions and lower the pH.

11. What is true about the relationship between [H+(aq)] and pH?

A. There is an inverse relationship; as [H+(aq)] decreases, the pH increases.
B. There is a direct relationship; as [H+(aq)] decreases, the pH decreases.
C. The relationship is complex. Because the concentrations are so small, the pH jumps after 200 ml of NaOH is added because small quantities of H+ ions increase the pH greatly, whereas large quantities of H+ ions have little effect.
D. There is no relationship between [H+(aq)] and pH.