Some solutes dissolve spontaneously in water even though the process absorbs heat and the solution cools. What favors the formation of these solutions?
An increase in the dispersal of matter
✓
A release of heat to the surroundings
Stronger attractions among solute particles
A decrease in the volume of the solvent
Correct Answer
An increase in the dispersal of matter
Dissolving spreads solute particles through the solvent, increasing the dispersal of matter (entropy), and that alone can make dissolution spontaneous even when it is endothermic. A release of heat to the surroundings would mean the process is exothermic, which contradicts the cooling. Stronger attractions among solute particles would make the solute harder to dissolve. A decrease in the volume of the solvent is not what drives solution formation.
Category: Dissolution Process
Question 2
In the three-step model of dissolution, which step releases energy?
Separating solute particles from one another
Forming attractions between solute and solvent particles
✓
Pushing solvent molecules apart to make room
Melting the solid solute before it mixes
Correct Answer
Forming attractions between solute and solvent particles
Forming attractions between solute and solvent particles (solvation) is exothermic, and whether the overall process absorbs or releases heat depends on how this energy compares with the first two steps. Separating solute particles from one another requires energy to overcome their attractions. Pushing solvent molecules apart to make room also requires energy. Melting the solid solute before it mixes is not a step in the model and would absorb energy anyway.
Category: Dissolution Process
Question 3
Which pair of substances forms a nearly ideal solution, mixing with essentially no energy change?
Sodium hydroxide and water
Ammonium nitrate and water
Pentane and hexane
✓
Calcium carbonate and water
Correct Answer
Pentane and hexane
An ideal solution forms with no energy change because the attractions between unlike particles are about the same as those between like particles, as with the two similar nonpolar hydrocarbons pentane and hexane. Sodium hydroxide and water mix with a large release of heat. Ammonium nitrate and water absorb heat on mixing, which is why the solution turns cold. Calcium carbonate and water barely form a solution at all, because the ionic attractions in the solid are too strong.
Category: Dissolution Process
Question 4
Solutions of four solutes, all at the same concentration in water, are tested with a light bulb conductivity apparatus. Which solution makes the bulb glow most brightly?
Acetic acid
Ethanol
Sucrose
Potassium chloride
✓
Correct Answer
Potassium chloride
Potassium chloride is an ionic compound that dissociates essentially completely into K+ and Cl- ions, so it is a strong electrolyte and gives a bright bulb. Acetic acid is a weak acid that ionizes only slightly, so the bulb glows dimly. Ethanol dissolves as neutral molecules, so it is a nonelectrolyte and the bulb stays dark. Sucrose also dissolves as neutral molecules and does not conduct.
Category: Electrolytes
Question 5
Pure hydrogen chloride gas contains no ions, yet its solution in water conducts electricity very well. How is HCl classified when dissolved in water?
A strong electrolyte
✓
A weak electrolyte
A nonelectrolyte
A colloidal dispersion
Correct Answer
A strong electrolyte
HCl is a strong acid: it reacts with water essentially completely to form H3O+ and Cl- ions, making it a strong electrolyte. A weak electrolyte produces ions from only a small fraction of its dissolved particles, which does not match a very good conductor. A nonelectrolyte produces no ions at all. A colloidal dispersion involves particles much larger than individual ions or molecules, and HCl in water is a true solution.
Category: Electrolytes
Question 6
Which substance dissolves in water as intact, uncharged molecules rather than as ions?
Sodium chloride
Sucrose
✓
Potassium dichromate
Sodium hydroxide
Correct Answer
Sucrose
Sucrose (table sugar, C12H22O11) is a covalent molecular compound whose molecules simply disperse among water molecules without forming ions. Sodium chloride is ionic and dissociates into Na+ and Cl- ions. Potassium dichromate is ionic and dissociates into K+ and Cr2O7 2- ions. Sodium hydroxide is ionic and dissociates into Na+ and OH- ions.
Category: Electrolytes
Question 7
The Henry's law constant for O2 in water at 25 °C is 1.3 x 10^-3 mol/(L·atm). What is the solubility of O2 when its partial pressure above the water is 0.20 atm?
6.5 x 10^-3 mol/L
1.3 x 10^-3 mol/L
2.6 x 10^-4 mol/L
✓
2.6 x 10^-3 mol/L
Correct Answer
2.6 x 10^-4 mol/L
Henry's law gives the solubility as C = kP, so C = (1.3 x 10^-3 mol/(L·atm))(0.20 atm) = 2.6 x 10^-4 mol/L. The value 6.5 x 10^-3 mol/L comes from dividing k by the pressure instead of multiplying. The value 1.3 x 10^-3 mol/L is the solubility at 1 atm and ignores the actual partial pressure. The value 2.6 x 10^-3 mol/L has the right digits but is off by a factor of 10.
Category: Solubility
Question 8
Warm water discharged from a power plant can cause fish kills in a river. Which property of the warmer river water is responsible?
It dissolves more oxygen
It has a higher osmotic pressure
It has a lower vapor pressure
It dissolves less oxygen
✓
Correct Answer
It dissolves less oxygen
The solubility of gases in water generally decreases as temperature rises, so warmer water holds less dissolved oxygen, and fish such as trout can suffocate; this is called thermal pollution. The claim that it dissolves more oxygen reverses the actual temperature trend. A higher osmotic pressure is not caused by warming water and does not explain the fish deaths. A lower vapor pressure is wrong because warming a liquid raises its vapor pressure.
Category: Solubility
Question 9
A small crystal of solute is dropped into a solution held at constant temperature. The crystal neither dissolves nor grows. How is the solution described?
Saturated
✓
Unsaturated
Supersaturated
Colloidal
Correct Answer
Saturated
A saturated solution already contains as much solute as its solubility allows, so it is in equilibrium with solid solute and there is no net dissolving or crystallizing. In an unsaturated solution the crystal would dissolve. In a supersaturated solution the crystal would act as a seed and solute would crystallize onto it. Colloidal describes a dispersion of large particles, not the amount of dissolved solute relative to its solubility.
Category: Solubility
Question 10
Ethanol and water can be mixed in any proportion to form a single homogeneous liquid. Which term describes this pair of liquids?
Immiscible
Miscible
✓
Partially miscible
Supersaturated
Correct Answer
Miscible
Liquids that mix with each other in all proportions are miscible; polar ethanol and polar water illustrate the rule that like dissolves like. Immiscible liquids, such as oil and water, have very low mutual solubility. Partially miscible liquids, such as bromine and water, dissolve in each other only moderately and form two layers. Supersaturated describes a solution holding more solute than its solubility, not a pair of liquids.
Category: Solubility
Question 11
A solution is made by dissolving 0.50 mol of glucose in 250 g of water. What is the molality of the solution?
0.13 m
0.50 m
2.0 m
✓
0.0020 m
Correct Answer
2.0 m
Molality is moles of solute per kilogram of solvent: 0.50 mol divided by 0.250 kg equals 2.0 m. The value 0.13 m comes from multiplying the moles by the mass in kilograms instead of dividing. The value 0.50 m treats the moles as if they were already in 1 kg of water. The value 0.0020 m comes from dividing by 250 without converting grams to kilograms.
Category: Concentration Units
Question 12
Why is molality, rather than molarity, used in boiling point elevation and freezing point depression calculations?
It is based on the volume of the solution
It counts only the ions in the solution
It has no units
It does not change with temperature
✓
Correct Answer
It does not change with temperature
Molality is based on the mass of solvent, and mass does not change as a solution is heated or cooled, so molality stays constant over the temperature range being studied. Being based on the volume of the solution describes molarity, and volume changes with temperature. Molality counts all dissolved solute, not only ions. Molality has units of mol/kg; a ratio such as mole fraction is the kind of concentration measure that has no units.
Category: Concentration Units
Question 13
A solution contains 0.50 mol of ethylene glycol, a nonelectrolyte, dissolved in 1.00 kg of water. Using Kf = 1.86 °C/m for water, what is the freezing point of the solution? Round to two decimal places.
-0.93 °C
✓
-1.86 °C
0.93 °C
-0.26 °C
Correct Answer
-0.93 °C
The molality is 0.50 mol / 1.00 kg = 0.50 m, so the freezing point depression is (1.86 °C/m)(0.50 m) = 0.93 °C, and the solution freezes at 0.00 - 0.93 = -0.93 °C. The value -1.86 °C uses Kf without multiplying by the molality. The value 0.93 °C adds the change instead of subtracting it, but a solute always lowers the freezing point. The value -0.26 °C uses the boiling point constant, 0.512 °C/m, by mistake.
Category: Colligative Properties
Question 14
A solution is 0.50 m NaCl in water. Assuming NaCl dissociates completely into Na+ and Cl- ions and using Kb = 0.512 °C/m, what is the boiling point of the solution? Round to two decimal places.
100.26 °C
100.51 °C
✓
101.86 °C
99.49 °C
Correct Answer
100.51 °C
Each formula unit of NaCl gives two ions, so the particle molality is 2 x 0.50 m = 1.0 m, and the boiling point rises by (0.512 °C/m)(1.0 m) = 0.512 °C to about 100.51 °C. The value 100.26 °C ignores dissociation and uses only 0.50 m of particles. The value 101.86 °C uses the freezing point constant, 1.86 °C/m, instead of the boiling point constant. The value 99.49 °C subtracts the change, but a nonvolatile solute raises the boiling point.
Category: Colligative Properties
Question 15
Assuming each ionic compound dissociates completely, which 0.10 m aqueous solution has the lowest freezing point?
Glucose
Sodium chloride
Calcium chloride
✓
Ethylene glycol
Correct Answer
Calcium chloride
Freezing point depression depends on the total concentration of dissolved particles, and CaCl2 gives three ions (Ca2+ and two Cl-) per formula unit, for 0.30 m of particles. Glucose is a nonelectrolyte, giving only 0.10 m of particles. Sodium chloride gives two ions per formula unit, or 0.20 m of particles. Ethylene glycol is a nonelectrolyte, giving only 0.10 m of particles.
Category: Colligative Properties
Question 16
What is the osmotic pressure of a 0.10 M aqueous glucose solution at 27 °C (300 K)? Use R = 0.08206 L·atm/(mol·K) and round to two significant figures.
25 atm
0.25 atm
1.2 atm
2.5 atm
✓
Correct Answer
2.5 atm
Osmotic pressure is MRT = (0.10 mol/L)(0.08206 L·atm/(mol·K))(300 K) = 2.46 atm, which rounds to 2.5 atm. The value 25 atm is too large by a factor of 10, a decimal error. The value 0.25 atm is too small by a factor of 10. The value 1.2 atm is half the correct result, as if the solute concentration were halved.
Category: Colligative Properties
Question 17
A solution of a nonvolatile solute in water has a water mole fraction of 0.90. Pure water at this temperature has a vapor pressure of 24 torr. What is the vapor pressure of the solution? Round to two significant figures.
22 torr
✓
2.4 torr
27 torr
24 torr
Correct Answer
22 torr
By Raoult's law, the vapor pressure of the solution is the solvent mole fraction times the pure solvent vapor pressure: 0.90 x 24 torr = 21.6 torr, or 22 torr. The value 2.4 torr uses the solute mole fraction, 0.10, instead of the solvent's. The value 27 torr divides by the mole fraction, which wrongly makes the vapor pressure rise. The value 24 torr ignores the vapor pressure lowering caused by the solute.
Category: Colligative Properties
Question 18
A searchlight beam becomes visible as it passes through fog because the suspended droplets scatter the light. What is this scattering called?
Brownian motion
Tyndall effect
✓
Doppler effect
Photoelectric effect
Correct Answer
Tyndall effect
The Tyndall effect is the scattering of light by colloidal particles, which are large enough to scatter light while dissolved molecules in a true solution are not. Brownian motion is the random jiggling of tiny suspended particles, not light scattering. The Doppler effect is the shift in frequency of a wave from a moving source. The photoelectric effect is the ejection of electrons from a metal by light.
Category: Colloids
Question 19
A cloud consists of tiny liquid water droplets dispersed in air. What type of colloid is it?
Foam
Gel
Aerosol
✓
Sol
Correct Answer
Aerosol
An aerosol is a colloid of liquid droplets (or solid particles) dispersed in a gas, as in clouds, fogs, mists and sprays. A foam is gas dispersed in a liquid, such as whipped cream. A gel is a liquid dispersed throughout a solid network, such as gelatin. A sol is a solid dispersed in a liquid, such as starch in water.
Category: Colloids
Question 20
Detergents clean better than soaps in hard water. What property of the products that detergents form with calcium and magnesium ions explains this?
They are insoluble
They are colored
They are volatile
They are water-soluble
✓
Correct Answer
They are water-soluble
Detergents have a sulfate or sulfonate ionic end, and their compounds with Ca2+ and Mg2+ stay dissolved, so the detergent keeps working. Saying they are insoluble describes soaps, whose calcium and magnesium salts precipitate as scum. Saying they are colored is unrelated to how well a cleaning agent works. Saying they are volatile is wrong, since these ionic compounds do not evaporate readily.
Category: Colloids