Lesson: Chapter - 5

Explanations of Practice Questions

1.F, T

(Do not fill in CE.) The first statement is false—water is actually less dense in its solid, frozen form, which is not true of most liquids. This is due to the hydrogen bonds that form between water molecules in water’s liquid state. When the temperature of water becomes low enough that water freezes, the molecules are locked into a regular, crystal lattice structure in which the molecules are actually farther apart from each other than they are when water is in its liquid state. The second statement is true, but since the first is false, there is no need to fill in the CE oval.

2.D

You’ll need to use the periodic table you’re given during the exam to answer this question since you’ll have to find the masses of the gases. Remember that the density of a gas is measured in grams per liter and that you will calculate the volume using the standard for gases at STP: 22.4 L per mole. Finally, remember that the heavier the molecule, the more dense the gas. A Helium is only 4 g/mol, B argon is 40 g/mol, C carbon dioxide is 44 g/mol, D xenon is 131 g/mol, and E nitrogen (N₂) is diatomic, so this means it has a mass of 28 g/mol. The best choice is D, xenon.

3.C

This question appears to be very involved, so you might be tempted to use the ideal gas equation; however, the question only asks for the pressure of the dry gas. You’re given atmospheric pressure and water vapor pressure at the defined temperature, so all you need to do is subtract the water vapor from the atmospheric pressure: 758 mmHg - 22.4 mmHg = 735.6 mmHg; this matches C. Since you will not have a calculator, round numbers off: 760 - 20 = 740. The closest answer choice is C.

4.E

In this problem, the phase does not change; liquid water increases in temperature by 20.0°C. This will take energy—to calculate the energy, use the equation q = mC_p?DT. You have all the information that you need: Q = (50.0 g)(4.184 J/gºC) (20.0ºC). Again, make the math easy: approximate that 20 × 50 = 1000, then multiply by 4 to get 4000 J—the only answer choice that’s close is choice E.

5.C

A is the triple point, so this statement is true. B is the line that separates the solid and the liquid, so this is also a true statement. C is the answer. The slope of this line is not negative; it is positive. The second half of the statement is true: when there is a positive slope, the solid is more dense than the liquid. D is where the solid and vapor phase meet, so this statement is also true—sublimation and deposition are the changes of state associated with solids and gases. Finally, E is also a true statement—the line represents where the liquid meets the vapor, and so these changes of state could take place.

6.T, T

(Fill in CE.) The first statement is true: most ionic solids are characterized as having a high melting point. Ionic solids are also hard and brittle, with poor thermal and electrical conduction. The second statement is also true—ionic solids consist of positive and negative ions held together by electrostatic attractions, and you should fill in CE because the second statement is the reason for the first statement’s being true.

7.E

To answer this question, you must think about the relationship between pressure and volume. Boyle’s law states that if the pressure is increased, the volume will decrease if all other factors are held constant. This type of relationship is an inverse relationship, and choice E is the only graph that shows an inverse relationship.

8.D

The boiling point of a solution depends on the solution’s concentration in molality and the number of ions that exist in solution. First eliminate any answer choices that contain organic compounds, since carbon compounds are covalently bonded and do not disassociate into ions in solution; in this way you can eliminates choices C and E. Choice A would produce two ions in solution, choice B would produce three ions in solution, and choice D would produce three ions in solution. Now you must choose between B and D. Since the molality is greater in D and D produces the same number of ions, it would have the highest boiling point.

9.D

This question requires that you know the definition of molality: molality is the number of moles of solute per kilogram of solvent. Since moles are not directly measured, instead the mass of the solute, (I), would be the component measured in the laboratory. The mass of the solvent can be expressed in kilograms of solvent, or (II). The total volume, (III), is not needed to calculate molality—just molarity, so look for an answer that contains both (I) and (II); the answer is D.

10.A

This question asks you to calculate the molarity, which is the moles of solute per liter of solution. 80.0 grams of NaOH can be converted to moles by dividing 80 by 40 (the molecular weight of NaOH), and this gives you 2 moles. Now divide 2 by 2 to get 1.0 M, which is answer choice A.

11.C

This question in essence asks which of the answer choices is a strong electrolyte. Strong electrolytes are strong acids, strong bases, and soluble salts. Let’s go through the answer choices. Choice A is not a strong electrolyte—it’s an alcohol. Choice B is acetic acid, an organic acid, and is only a weak electrolyte. Choice C is a soluble salt; all nitrates are soluble and thus dissociate in solution. This is the correct answer. Choice D is ionically bonded but is not a strong base. In group 2A, only barium, strontium, and calcium hydroxides are considered to be strong. Mg(OH)₂ is a chalky white liquid often taken as a laxative.

12.T, F

(Do not fill in CE.) Statement I is true—when the pressure of a system is decreased, the solubility of a gas decreases; as you probably know from experience, when you open a soda bottle, the pressure of the system decreases significantly, and gas bubbles rise to the top and escape. Statement II says that heating a gas in a solution will make the gas more soluble; this is not true. Since this statement is false, you would not fill in the CE oval.

13.E

This is a dilution problem, so use the formula you learned in this chapter to solve it: M₁V₁ = M₂V₂. The problem gave you the following values to plug in: (6.0 M) (50.0 mL) = (M) (300.0 mL). Do the math, and you get 300 divided by 300 = 1.0 M. This is answer choice E.

14.E

When you see a problem that looks like this, inspect each pairing one at a time. Here keep in mind that “like dissolves like,” so polar substances will be soluble in polar and ionic substances, and nonpolar substances will be soluble in nonpolar substances. Look at (I): ethanol is a polar substance due to the hydroxyl group, and water is also polar, so these two substances should make a stable solution. (II) Salt is ionically bonded and water is polar, so these two should also make a stable solution. (III) Oil is nonpolar, and vinegar (acetic acid) is polar because of its carboxylic acid group, so these two substances should not mix well. (IV) Oil and gasoline are both nonpolar, so these two substances should make a stable solution. Look for the answer choice that contains I, II, and IV; choice E is the match.

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