MCAT General Chemistry Practice Test 5: Gases

The earth's atmosphere has several layers of gases with different characteristics and temperatures. The atmosphere has a total mass of 5 × 10¹⁸ kg, and is composed mainly of nitrogen, oxygen, argon, and carbon dioxide, as well as water vapor to a variable degree. Seventy-five percent of this mass is within 11 km from the earth's surface. Although the limit between outer space and the atmosphere is not definite, the Kármán Line, approximately 100 km above sea level, is often taken as the boundary.

Table 1 Composition of atmosphere (dry)

The five main layers of atmosphere are the exosphere, thermosphere, mesosphere, stratosphere, and troposphere. The troposphere is the innermost layer, and extends to approximately 6 km above the earth's surface at the poles and approximately 20 km above sea level at the equator. The next layer is the stratosphere. The ozone layer, which is considered to be a layer of its own because of its unique composition, is within the stratosphere. Approximately 90 percent of the O₃ in the atmosphere is found in the ozone layer, although the actual concentrations are quite low (2-8 ppm). This layer is very important as it absorbs much of the ultraviolet (UV) light emitted from the sun. The mesosphere is the middle layer of the atmosphere with a temperature of approximately 100°C. The thermosphere and exosphere in that order are the outermost layers.

The pressure, density, and temperature of the atmosphere vary with altitude. Both pressure and density decrease with increasing altitude. At sea level, the density of air is 1.2 kg/m³ and drops by approximately 50 percent every 5.5 km.

The atmosphere contains greenhouse gases, which absorb and emit thermal infrared radiation, leading to the greenhouse effect. Greenhouse gases include water vapor, methane, nitrous oxide, carbon dioxide, and ozone. These gases are needed to maintain the temperature of the earth, which would otherwise be much colder. However, it is thought that an increase in the amount of greenhouse gases in the atmosphere has contributed to the increase in average temperature in the twentieth century. Methane is a very potent greenhouse gas. It can be oxidized in the atmosphere to produce carbon dioxide and water, with a half-life of 7 years.

1. How long would it take for a 1 L sample of methane to decrease to 1 percent of its original amount from atmospheric oxidation?

• A. 38 years
• B. 42 years
• C. 47 years
• D. 50 years

2. What is the partial pressure of nitrogen gas in the atmosphere at sea level?

• A. 7.91 × 10¹ Pa
• B. 5.96 × 10² Pa
• C. 7.91 × 10⁴ Pa
• D. 5.96 × 10⁵ Pa

3. The summit of Mount Everest is 8.8 km above sea level. What is the approximate density of air at this point?

• A. 0.2 kg/m³
• B. 0.4 kg/m³
• C. 0.8 kg/m³
• D. 1.0 kg/m³

4. A sample of gas containing oxygen, nitrogen, argon, and carbon dioxide in equal molar proportions is in a closed container. Which of these gases would escape the fastest if a small hole were punctured in the container?

• A. Nitrogen
• B. Oxygen
• C. Argon
• D. Carbon dioxide

5. All of the following are true regarding the earth’s atmosphere EXCEPT:

• A. the boundary between the atmosphere and outer space is indistinct.
• B. the ozone layer is primarily composed of ozone gas.
• C. more than 3.75 × 10¹⁸ kg of atmospheric gases is contained within an 11 km distance from the earth's surface.
• D. water vapor contributes to warmer atmospheric temperatures.

6. If a 20 L sample of gas at STP were cooled to mesosphere temperatures at a constant volume, what would the new pressure be?

• A. 64 kPa
• B. 84 kPa
• C. 96 kPa
• D. 128 kPa