Kaplan MCAT General Chemistry Review

Gases deviate from ideal behavior at higher pressures, which forces molecules closer together. The closer they are, the more they can participate in intermolecular forces, which violate the definition of an ideal gas. As the temperature of a gas is reduced, the average velocity of the gas molecules decreases, and the attractive intermolecular forces become more significant. This results in the loss of another characteristic of an ideal gas, and thus less ideal behavior. Answer choices that include the opposite, high temperature and low pressure, are incorrect because gases behave most ideally under these conditions. At high temperatures, molecules will move quickly and exhibit random motion and elastic collisions, which is a property of ideal gases. In an ideal gas it is assumed that there are no intermolecular attractions between gas molecules, which is valid at low pressures when there is ample space between them.

2. D

Density equals mass divided by volume. The mass of 1 mole of neon gas equals 20.18 grams. At STP, 1 mole of neon occupies 22.4 L. Dividing the mass, 20.18 grams, by the volume, 22.4 L, gives an approximate density of 0.9009 g L^-1.

3. C

Graham’s law of effusion states that the relative rates of effusion of two gases at the same temperature and pressure are given by the inverse ratio of the square roots of the masses of the gas particles. In equation form, Graham’s law can be represented by: Rate₁ /Rate₂ = . If a molecule has a higher molecular weight, then it will leak at a slower rate than a gas with a lower molecular weight. Both neon and oxygen gases will leak at slower rates than helium because they both weigh more than helium.

4. B

The pressure of the gas is calculated by subtracting the vapor pressure of water from the measured pressure during the experiment: 784 mm Hg - 24 mm Hg = 760 mm Hg, or 1 atm. The ideal gas law can be used to calculate the moles of hydrogen gas. The volume of the gas equals 0.100 L, the temperatures equals 298 K, and R = 0.0821 (L atm / mol K). Solving the equation PV = nRT for n gives 4.09 × 10^-3 moles of hydrogen. (A) is incorrect and would result from mistakenly using 784 mm Hg of the pressure instead of the pressure adjusted for water vapor. (C) and (D) would both result from incorrectly using a pressure in mm Hg instead of converting to atm while using the gas constant R = 0.0821 (L atm / mol K).

5. C

Ideal gases are said to have no attractive forces between molecules. They are considered to have point masses, which theoretically take up no volume.

6. C

The first thing to do is balance the given chemical equation. The coefficients, from left to right, are 1, 1, and 2. The mass of solid, 8.01 grams, can be converted to moles of gas product by dividing by the molar mass of NH₄NO₃(s) (80.06 g) and multiplying by the molar ratio of 3 moles of gas product to one mole of NH₄NO₃(s). This gives approximately 0.300 moles of gas product. The ideal gas equation can be used to obtain the pressure in the flask. The values are as follows: R equals 0.0821 (L atm / mole K), the temperature in Kelvin is 500 K, and the volume is 10.00 L. Solving for P in the equation PV = nRT gives a pressure of about 1.23 atm.

7. A

The average kinetic energy is directly proportional to the temperature of a gas in Kelvin. The kinetic molecular theory states that collisions between molecules are elastic and thus do not result in a loss of energy. The kinetic energy of each gas molecule is not the same.

8. C

At STP, the difference between the distribution of velocities for helium and bromine gas is due to the difference in molar mass (Rate /Rate ) = . Helium has a smaller molar mass than bromine. Particles with small masses travel faster than those with large masses, so the helium gas corresponds to curve B with higher velocities. (A) and (B) are incorrect because they inaccurately identify each curve on the graph. Given that the gases are at the same temperature (STP), we can recall that temperature relates to kinetic energy (KE). KE = kT . The gases average KE, therefore should be the same. Therefore, answer (D) is also incorrect.

9. D

At STP, the pressure inside the balloon equals 1 atm. The total number of moles in the balloon equals 0.20 moles plus 0.60 moles, or 0.80 moles. P_O2 equals the mole fraction of oxygen (0.20/0.80) times the total pressure, 1 atm. The partial pressure of oxygen is 0.25 atm.

10. A

The ideal gas law can be modified to include density and determine the temperature of the sun.

n = mass/molecular weight density (denoted D for this problem) = mass/V

The density is given in g/cm³ and must be converted to g/L so the units cancel in the above equation. Because 1 cm³ equals 1 mL and there are 1,000 mL in 1 L, the density can be multiplied by 1,000 to be converted to g/L.

11. A