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MCAT Sample Questions : Physical Sciences

Passage II

At the critical point, the density of liquid CO2 is equal to the density of gaseous CO2.This occurs at specific conditions of temperature and pressure.At temperatures and pressures above the critical point values, CO2 is deemed supercritical.For a supercritical fluid, the density and ability to dissolve other substances are similar to values expected for liquids.The following figures give phase data for CO2.

Figure 1 Phase diagram for CO2

Figure 2 Isothermal curves for CO2 near the critical point.V (x 104) is relative to V = 1.000 at 0oC and 1.000 atm.

Some sample questions on this passage are as follows:

1. In Figure 2, which of the points (A-D) is the critical point for CO2?

1. A
2. B
3. C
4. D

Explanation: The liquid and vapor phases coalesce at point D of Figure 2, where the densities of liquid and gaseous CO2are equal. Thus, answer choice D is the best answer

2. In an extraction of an organic oil, which of the following is an advantage of using supercritical CO2?

1. It reacts with most organic compounds.
2. It is easily handled at room temperature.
3. It crystallizes easily.
4. It is easily removed by evaporation

Explanation:The question does not compare CO2 to a specific solvent, so we are looking for an inherent property of CO2 that makes it a good solvent for an organic oil. Supercritical CO2 is similar to a liquid and can be easily removed by evaporation because it changes into a gas when the pressure is lowered. Answers A, B, and C are not true of CO2, and answers A and C are not desirable properties of an extraction solvent. Thus, answer choice D is the best answer

3. Water is a liquid at room temperature, yet CO2 at room temperature is liquid only at high pressures.Which of the following best explains this?

1. CO2 is polar and has strong intermolecular forces.
2. CO2 is nonpolar and has strong intermolecular forces.
3. CO2 is polar and has weak intermolecular forces.
4. CO2 is nonpolar and has weak intermolecular forces

Explanation: Polar water molecules are held together by relatively strong hydrogen bonds; whereas, the linear, nonpolar molecules of CO2 are held together at room temperature by weak London dispersion forces. Thus, answer choice D is the best answer.

4. According to Figure 1, what is the critical temperature and pressure of CO2?

1. –56.1oC and 6.0 atm
2. –54.1oC and 119.0 atm
3. 31.1oC and 75.3 atm
4. 25.0oC and 1.0 atm

Explanation:The critical point, shown as a dot (·) in Figure 1, is near 30oC and 80 atm. Answer choice C is the best answer

5. Which of the following compounds is most soluble in supercritical CO2?

1. NaCl
2. C2H5OC2H5
3. NH4NO3
4. KOH

Explanation: According to the principle of “like dissolves like,” the covalent compound CO2 is a better solvent for a covalent compound than it is for an ionic compound. Diethyl ether, C2H5OC2H5, is a covalent compound and NaCl, NH4NO3, and KOH are ionic compounds. Thus, answer choice B is the best answer.

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