1. Which substance is most likely to be a solid at STP?
2.Which of the following tend to increase the melting point of a solid?
I. Increasing molecular weight
II. Decreasing polarity
III. Increasing surface area
A. I and II
D. I and III
3. A gas at constant volume is cooled. Which statement about the gas must be true?
A. The kinetic energy of the gas molecules has decreased.
B. The gas has condensed to a liquid.
C. The weight of the gas has decreased.
D. The density of the gas has increased.
4. A weather balloon is filled with 1000 mol of He gas at 25 °C and 101 kPa of pressure. What is the volume of the weather balloon?
A. 24518 m3
B. 24.5 m3
C. 2 m3
D. 245 m3
5. One mole of oxygen gas and two moles of hydrogen are combined in a sealed container at STP. Which of the following statements is true?
A. The mass of hydrogen gas is greater than the mass of oxygen.
B. The volume of hydrogen is greater than the volume of oxygen.
C. The hydrogen and oxygen will react to produce 2 mol of water.
D. The partial pressure of hydrogen is greater than the partial pressure of oxygen.
6. Graham’s law is best used to determine what relationship between two different materials?
A. pressure and volume
B. volume and temperature
C. mass and diffusion rate
D. Diffusion rate and temperature
7. Which is the correct order of increasing intermolecular attractive forces?
A. Dipole-dipole < ionic
B. Ionic < dipole-dipole
C. Hydrogen bonding
D. London dispersion < dipole-dipole
8. One mole of an ideal gas is compressed to 10 L and heated to 25 °C. What is the pressure of the gas?
A. 2.4 KPa
B. 2.4 atm
C. 0.2 atm
D. 0.2 KPa
9. A 10 L cylinder contains 4 moles of oxygen, 3 moles of nitrogen and 7 moles of neon. The temperature of the cylinder is increased from 20 °C to 40 °C. Determine the partial pressure of neon in the cylinder as a percentage of the whole.
10. Three liquids, X, Y and Z are placed in separate flasks, each of which is suspended in a water bath at 75 °C. The boiling points of each liquid are
X, 273 K
Y, 340 K
Z, 360 K
Which of the three liquids will begin to boil after warming to 75 °C?
A. X, Y, and Z
B. X and Z
C. X and Y
D. Y and Z
ANSWER KEY EXAM 240
1. Answer: B
Na (sodium) is a solid at standard temperature and pressure, which is 0oC (273 K) and 100 kPa (0.986 atm), according to IUPAC. The stronger the intermolecular forces, the greater the likelihood of the material being a solid. Kr and Xe are noble gases and have negligible intermolecular attraction. NH3 has some hydrogen bonding but is still a gas at STP. Sodium is an alkali metal whose atoms are bonded by metallic bonding and is therefore a solid at STP.
2. Answer: D
Generally, the larger and heavier the molecule, the higher the melting point. Decreasing polarity will lower intermolecular attractions and lower the melting point. Long, linear molecules have a larger surface area, and therefore more opportunity to interact with other molecules, which increases the melting point.
3. Answer: A
The kinetic energy of the gas molecules is directly proportional to the temperature. If the temperature decreases, so does the molecular motion. A decrease in temperature will not necessarily mean a gas condenses to a liquid. Neither the mass nor the density is impacted, as no material was added or removed, and the volume remained the same.
4. Answer: B
The ideal gas law PV=nRT is rearranged to solve for V, and we get V = nRT/P. R is the gas constant, 0.08206 L atm/mol K, and the Celsius temperature must be converted to Kelvin, by adding 273 to 25oC to obtain 298 K. The pressure must be converted to atmospheres, which 101 kPa is essentially 1 atm (0.9967 atm). Plugging the numbers into the equation we get V = 1000 mol (0.08206 L atm/mol K)(298 K)/1 atm, which gives V = 24,453 L. A liter is a cubic decimeter (dm3) and when converted gives V = 24.5 m3.
5. Answer: D
Since there are twice as many molecules of hydrogen present vs. oxygen, the partial pressure of hydrogen will be greater. The mass of hydrogen will not be greater than the mass of oxygen present even though there are more moles of hydrogen, due to oxygen having a higher molecular weight. Each gas will occupy the same volume. Hydrogen and oxygen gas can coexist in the container without reacting to produce water. There is no indication given that a chemical reaction has occurred.
6. Answer: C
Graham’s law of diffusion allows one to calculate the relative diffusion rate between two different gases based on their masses.
7. Answer: D
London dispersion forces are the weakest intermolecular forces. These interactions occur in all molecules due to unequal electron density around the nucleus, which results in a momentary dipole. Dipole-dipole interactions are those between two polar molecules. The more positive portion of one molecule is attracted to the negative portion of a different molecule. Hydrogen bonding is a stronger type of dipole-dipole interaction which occurs between a hydrogen in one molecule and a nitrogen, oxygen or fluorine atom in another molecule. Hydrogen bonding only occurs between molecules containing H-F, H-O or H-N bonds. Ionic bonds are the strongest intermolecular forces. In ionic molecules, a positive ion is attracted to a negative ion. NaCl is entirely ionic with full charge separation, and the ions are tightly bound to each other in an organized crystalline network.
8. Answer: B
Plugging the data into the ideal gas law using the correct units gives the correct answer in atmospheres, which in this case is 2.4 atm. The equation is P = nRT/V. So we have P = 1 mol (0.08206 L atm/mol K)(298 K)/10 L. The R value is 0.08206 L atm/mol K when using L as the volume unit, and delivers the pressure in atm.
9. Answer: A
Since there are 7 moles of neon out of a total of 14 moles of gas in the cylinder, the partial pressure of neon will always be 50% of the total pressure, regardless of the temperature.
10. Answer: C
To convert from degrees Celsius to Kelvin, add 273. 75° C is equivalent to 348 K. Both X and Y have lower boiling points, which means that they will each boil in the water bath. Z will never become warm enough to boil.