Question

Which aqueous solution from the column on the right has the property listed on the left? Explain your choices. $$\begin{array}{ll}\hline \text { Property } & \text { Solution } \\\\\hline \text { 1. lowest electrical } & \text { a. } 0.10 \mathrm{m} \mathrm{KCl}(\mathrm{aq}) \\\\\text { conductivity } & \\\\\text { 2. } \text { lowest boiling } & \text { b. } 0.15 \mathrm{m}\mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{11}(\mathrm{aq}) \\\\\text { point } & \\\\\text { 3. highest vapor pressure } & \text { c. } 0.10 \mathrm{m} \mathrm{CH}_{3} \mathrm{COOH}(\mathrm{aq}) \\\\\text { of water at } 25^{\circ} \mathrm{C} & \\\\\text { 4. lowest freezing point } & \text { d. } 0.05 \mathrm{m} \mathrm{NaCl} \\\\\hline\end{array}$$

Short answer

The solution with the lowest electrical conductivity is c. CH3COOH(aq). The solution with the lowest boiling point andhighest vapor pressure is b. C12H22O11(aq). The solution with the lowest freezing point is d. NaCl. These conclusions are derived from the nature of these compounds and their behaviors when dissolved in water.

Step-by-step solution

Lowest Electrical Conductivity

The solution with the least amount of ions or with the ions that do not ionize completely should have the lowest electrical conductivity. Among the given solutions, CH3COOH (acetic acid) is a weak acid and does not ionize completely in water, whereas other solutions are either ionic salts or sucrose that ionize completely or do not ionize at all. Hence, solution c, CH3COOH should have the lowest electrical conductivity.

Lowest Boiling Point

The solution that interferes least with the intermolecular forces of water will have the lowest boiling point. The only solution that does not introduce any ions and thus preserves waters' boiling point as much as possible is b. C12H22O11(aq). Boiling point elevation is a colligative property, which depends upon the number of solute particles in solution rather than their nature. As a molecular compound, C12H22O11 does not break apart into ions when it dissolves and thus has the least effect

Highest Vapor Pressure of Water

Again, the solution which interferes least with the intermolecular forces of water will have the highest vapor pressure. Therefore, the option is again b. C12H22O11. Sucrose does not ionize in water when it dissolves. It therefore has less of an effect on the vapor pressure than the other solutes that ionize into at least two particles when they dissolve.

Lowest Freezing Point

The solution with more ions will have the lowest freezing point. Both NaCl and KCL dissociate into two ions when they dissolve, but NaCl is present at lower molarity. Still, it provides more ions in solution because in water it produces 2 ions, while sugar does not ionize and CH3COOH being a weak acid ionizes only to a small extent. Therefore, solution d, 0.05 m NaCl, should have the lowest freezing point, because it produces the most particles (ions) per formula unit of solute.

Key concepts

Electrical Conductivity

Electrical conductivity in solutions is all about how easily electricity can travel through the solution. When we dissolve a substance in water, it may break into pieces called ions, which can move and carry electricity. Substances that break apart a lot and make many ions are great at conducting electricity.

In our example, we have different substances dissolved in water, like KCl, CH3COOH, and NaCl. KCl and NaCl are ionic compounds and break into ions completely. CH3COOH, on the other hand, is a weak acid and doesn't break up much; this means fewer ions to carry electricity. Therefore, CH3COOH has the lowest electrical conductivity among the solutions. This is because with fewer ions, the pathway for electricity isn't as strong.

Boiling Point

The boiling point of a liquid is the temperature at which it turns into vapor. When we add a solute to water, it affects when water will start turning into vapor. This effect is called boiling point elevation, which is a colligative property.

While some substances add many particles or ions into the water when they dissolve, others do not. In simpler terms, the more solute particles, the higher the boiling point. Sucrose, a sugar, doesn’t break into ions in water, so it interferes the least with water’s boiling point. That's why the solution with sucrose has the lowest boiling point compared to others, which introduce more particles.

Vapor Pressure

Vapor pressure is the measure of how much of a liquid's molecules can escape into the air as vapor. Solutions can change the vapor pressure by how they interact with the water. Colligative properties again play a role here; they depend on the number of particles in the solution rather than just their type.

Sucrose is quite different from salts when dissolved in water. It doesn't produce ions, thus interfering the least with water's tendency to evaporate. With sucrose, the amount of vapor produced above the liquid - the vapor pressure - remains higher. Therefore, sucrose solution has a higher vapor pressure than solutions with more solute particles present.

Freezing Point

The freezing point is the temperature at which a liquid becomes a solid. Adding solute to water can lower its freezing point, a phenomenon known as freezing point depression. The key idea here is that more particles make it harder for water to freeze.

In our exercise, different solutions are created, each containing varying numbers of particles when dissolved. NaCl breaks into two ions, Na+ and Cl-, increasing the number of particles significantly. Even though it is a lower molarity solution, it still provides enough ions to lower the freezing point most effectively. Hence, 0.05 m NaCl has the lowest freezing point due to the presence of numerous ions which disrupt water's structure while freezing.