Chapter 12

Some alcohols are quite soluble in water. For example, isopropyl alcohol, shown below, is sold as an aqueous solution we call rubbing alcohol. Show how a water molecule would be attracted to isopropyl alcohol, and name the strongest intermolecular force involved. Would you call this interaction hydration?

Why is the amount of energy associated with the solvation step of the dissolving process so critical to whether or not a solute will dissolve?

Consider an ionic solid dissolving in a liquid. Suppose the energy released as solute-solvent interactions take place is substantially less than the energy required to break up the lattice and to make room in the solvent. Would the solid be very soluble in this liquid? Explain your answer fully.

Consider dissolving an ionic salt in roomtemperature water. (a) Some salts dissolve in water to yield a much warmed solution. How is this possible? (b) Some salts dissolve in water to yield a cold solution. How is this possible?

When solid sodium hydroxide \(\mathrm{NaOH}(\mathrm{s})\) pellets are dissolved in water, the water can get extremely hot and almost boil to give \(\mathrm{NaOH}(a q)\) solution. What is the source of this heat?

Suppose the hydration energy for an ionic compound is much less than the energy required to pull apart the lattice. Is such a compound likely to be soluble or insoluble in water? Explain your answer.

Sodium chloride is very soluble in water but insoluble in liquid hexane, \(\mathrm{C}_{6} \mathrm{H}_{14} .\) Why is this so?

Even though NaCl does not dissolve in hexane, \(\mathrm{C}_{6} \mathrm{H}_{14}\), if we imagine the dissolving process for this system, there is one step that would take less energy than the corresponding step for \(\mathrm{NaCl}\) dissolving in water. Which step is it? Why would it require less energy?

Which has the greater hydration energy, \(\mathrm{AlCl}_{3}\) or NaCl? Explain your answer.

In theory, it is possible for one ionic compound to have a greater hydration energy than another and still be the less soluble of the two. What would have to be true for this to be so?

What is meant by the rule of thumb "like dissolves like"?

What do we mean by the total energy change \(\Delta E_{\text {total }}\) for the dissolving process, and why is it important to know about this energy change?

Suppose you have two ionic compounds, A and B. The only significant difference between them is that the ionic bonds that hold the lattice together in compound \(\mathrm{A}\) are stronger than those in compound B. Which compound would you expect to be more soluble in water? Explain your answer.

When a gaseous solute dissolves in water, which step in the dissolving process is essentially skipped? Explain why.

When a liquid solute dissolves in water, there is still a solute-separation step that absorbs energy, but the step doesn't require breaking up a crystal lattice as for a typical solid solute. What happens in this step with a liquid solute, and why does the step absorb energy?

Oil is insoluble in water. Which step in the threestep process for dissolving is responsible for this? Explain why this step is the problem.

What is entropy?

The natural tendency for systems is to decrease in energy. What is the natural tendency for entropy?

When you release a drop of blue food dye into a beaker of water, the drop eventually dissolves to give a homogeneous light-blue solution. No matter how long you wait, the dye molecules will never regroup to form the original drop. Why not?

In terms of total energy change, when is entropy an important factor in determining whether or not a solute dissolves in a solvent? When is it not an important factor?

True or false? Whenever something spontaneously dissolves in water, the entropy has increased. Justify your answer.

Certainly if NaCl dissolved in liquid hexane, \(\mathrm{C}_{6} \mathrm{H}_{14}\), the entropy associated with the \(\mathrm{Na}^{+}\) and \(\mathrm{Cl}^{-}\) ions would increase. Nevertheless, \(\mathrm{NaCl}\) does not dissolve in hexane. Why not?

How does increasing temperature affect the solubility in water (a) of most solids and (b) of gases?

What effect does increasing pressure have on the solubility of gaseous solutes? Explain your answer.

How is the medical condition known as the bends related to solubility?

Aquatic life is often damaged when hot water is discharged from power stations into rivers and lakes. What might this have to do with gas solubility in water?

When a can of soda pop is opened, the pressure of carbon dioxide gas that is above the pop in the can drops from high pressure to \(1 \mathrm{~atm}\). What does this have to do with the pop going flat when the can is left open?

(a) Use these data to plot solubility as a function of temperature for \(\mathrm{KCl}\) and \(\mathrm{Li}_{2} \mathrm{SO}_{4}:\) (b) Using the plot, estimate the solubility of both compounds in water at \(70^{\circ} \mathrm{C}\). (c) How much of each compound can be dissolved in a beaker containing \(75 \mathrm{~g}\) of water at \(70^{\circ} \mathrm{C} ?\)

Define molarity.

Give precise instructions to your laboratory assistant as to how to prepare \(1.00 \mathrm{~L}\) of a \(1.00 \mathrm{M}\) aqueous solution of \(\mathrm{CaCl}_{2}\). Remember that your assistant will be measuring out the \(\mathrm{CaCl}_{2}\) in grams. She has available a 1-L volumetric flask.

Give precise instructions to your laboratory assistant as to how to prepare \(1.00 \mathrm{~L}\) of a \(1.00 \mathrm{M}\) aqueous solution of sucrose, \(\mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{11}\). Remember that she will be measuring out the sucrose in grams. She has available a 1-L volumetric flask.

Give precise instructions to your laboratory assistant as to how to prepare \(1.00 \mathrm{~L}\) of a \(0.250 \mathrm{M}\) aqueous solution of sucrose, \(\mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{11}\). Remember that she will be measuring out the sucrose in grams. She has available a 1-Lvolumetric flask.

Give precise instructions to your laboratory assistant as to how to prepare \(0.500 \mathrm{~L}\) of a \(1.50 \mathrm{M}\) aqueous solution of sucrose, \(\mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{11}\). Remember that she will be measuring out the sucrose in grams. She has available a 0.50-L volumetric flask.

Your assistant tells you she measured out \(2.50\) moles of \(\mathrm{NaCl}\) and then added enough water to get \(500.0 \mathrm{~mL}\) of solution to prepare a \(5.00 \mathrm{M}\) solution of \(\mathrm{NaCl}\). (a) What was the mass of the \(\mathrm{NaCl}\) in grams? (b) Did she successfully prepare a \(2.5 \mathrm{M}\) solution? Prove your answer.

Your assistant tells you he measured out \(116.886\) \(\mathrm{g}\) of \(\mathrm{NaCl}\) and then added exactly \(1.00 \mathrm{~L}\) of water to it to prepare a \(2.00 \mathrm{M}\) solution of \(\mathrm{NaCl}\). Do you fire him or give him a promotion? Explain.

You have \(2500.0 \mathrm{~mL}\) of a \(0.250 \mathrm{M}\) solution of \(\mathrm{NaCl}\). (a) How many moles of \(\mathrm{NaCl}\) are present in this solution? (b) How many moles of ions are present in this solution? (c) How many grams of \(\mathrm{NaCl}\) would you recover if you evaporated all of the water off of this solution?

You have \(45.0 \mathrm{~mL}\) of a \(0.250 \mathrm{M}\) solution of sucrose, \(\mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{11}\) (a) How many moles of \(\mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{11}\) are present in this solution? (b) How many grams of sucrose would you recover if you evaporated all of the water off of this solution? (c) A student says that if you did part (b) and recovered all of the evaporated water as a liquid, you would get \(45.0 \mathrm{~mL}\) of liquid water. Is this student correct? Explain.

How many milliliters of a \(1.00 \mathrm{M}\) solution of \(\mathrm{NaCl}\) are required to obtain \(5.00 \mathrm{~g}\) of \(\mathrm{NaCl} ?\)

How many milliliters of a \(0.250 \mathrm{M}\) solution of glucose, \(\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}\), are required to obtain \(100.0 \mathrm{~g}\) of glucose?

There is a bottle of \(0.500 \mathrm{M}\) sucrose stock solution in the laboratory. Give precise instructions to your assistant on how to use the stock solution to prepare \(250.0 \mathrm{~mL}\) of a \(0.348 \mathrm{M}\) sucrose solution.

There is a bottle of \(4.50 \mathrm{M} \mathrm{NaCl}\) solution in the laboratory. Give precise instructions to your assistant on how to use the stock solution to prepare \(100.0 \mathrm{~mL}\) of a \(4.00 \mathrm{M} \mathrm{NaCl}\) solution.

A student plans to divide the molarity of his solution by its volume to determine the number of moles of solvent in it. He is making two mistakes here. Identify the mistakes and correct both of them.

Consider the three types of percent composition of a solution. (a) What are the names of these three types of percent compositions? (b) Which term is missing from all three types of percent compositions: solvent, solute, or solution? (c) Give the mathematical definition of each type of percent composition.

In Practice Problem 12.3, you learned that "proof" for an alcoholic drink equals twice the percentage of alcohol in the drink. The complete definition of proof is that it is twice the percentage by volume of alcohol. Knowing this, exactly what does it mean to have a 90 -proof drink?

A solution of ethanol is prepared by combining \(22.5 \mathrm{~g}\) of ethanol with \(49.6 \mathrm{~g}\) of water. What is the percent composition by mass of alcohol in this solution?

A solution of a particular solid solute in water has a concentration of \(25.0\) mass \(\%\). (a) Given \(100.0 \mathrm{~g}\) of this solution, how many grams of solute do you have? (b) Given \(48.0 \mathrm{~g}\) of this solution, how many grams of solute do you have? (c) How many grams of this solution do you need to obtain \(56.5 \mathrm{~g}\) of solute?

How would you prepare \(2.00 \mathrm{~kg}\) of an \(\mathrm{NaCl}\) solution that is \(30.0\) mass \(\% \mathrm{NaCl} ?\)

You want \(1.00\) L of an alcohol-water solution that is \(5.00\) vol \(\%\) alcohol. (a) How would you prepare this solution? (b) Suppose the alcohol is ethanol, \(\mathrm{C}_{2} \mathrm{H}_{6} \mathrm{O}\), which has a density of \(0.789 \mathrm{~g} / \mathrm{mL}\) and an \(\mathrm{MM}=46.07 \mathrm{~g} / \mathrm{mol}\). What would be the molarity of the solution?

An alcohol-water solution is \(35.00\) vol \% alcohol. How much solution is required to obtain \(200.0\) \(\mathrm{mL}\) of alcohol?

An aqueous solution is \(25.0\) mass \% \(\mathrm{NaCl}\). The density of the solution is \(1.05 \mathrm{~g} / \mathrm{mL}\). What is the molarity of the solution?