Question

List the following aqueous solutions in order of decreasing freezing point: \(0.040 \mathrm{~m}\) glycerin \(\left(\mathrm{C}_{3} \mathrm{H}_{8} \mathrm{O}_{3}\right), 0.020 \mathrm{~m} \mathrm{KBr}\), \(0.030 \mathrm{~m}\) phenol \(\left(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH}\right)\)

Short answer

The order of the aqueous solutions in decreasing freezing point is: \(0.020\ \mathrm{m}\) KBr > \(0.040\ \mathrm{m}\) glycerin > \(0.030\ \mathrm{m}\) phenol.

Step-by-step solution

Identify the number of particles formed by each solute in water

First, we will identify the number of particles (ions or molecules) formed in water by each solute: 1. Glycerin (C3H8O3) - it is a molecular compound and will not form ions in water, so it forms 1 particle. 2. KBr - It is an ionic compound and will dissociate into 2 ions in water: K+ and Br-. 3. Phenol (C6H5OH) - it is a molecular compound and will not form ions in water, so it forms 1 particle.

Calculate the effective molality of particles for each solution

Next, we will calculate the effective molality of particles for each solution by multiplying the molality of the solute (given in the exercise) by the number of particles formed by that solute in water: 1. \(0.040\ \mathrm{m}\) glycerin: \(0.040\ \mathrm{m} \times 1 = 0.040\ \mathrm{m}\) particles 2. \(0.020\ \mathrm{m}\) KBr: \(0.020\ \mathrm{m} \times 2 = 0.040\ \mathrm{m}\) particles 3. \(0.030\ \mathrm{m}\) phenol: \(0.030\ \mathrm{m} \times 1 = 0.030\ \mathrm{m}\) particles

Compare the effective molalities to determine the order of freezing point depression

We will now compare the effective molalities of particles to determine the order of decreasing freezing point: A higher effective molality of particles results in a larger freezing point depression, which in turn corresponds to a lower freezing point. Based on the effective molalities calculated in Step 2, we can order the solutions as follows: 1. \(0.040\ \mathrm{m}\) KBr - KBr has the highest effective molality of particles and will have the lowest freezing point. 2. \(0.040\ \mathrm{m}\) glycerin - Glycerin has the same effective molality as KBr but was originally less concentrated, so its freezing point is higher than KBr but lower than phenol. 3. \(0.030\ \mathrm{m}\) phenol - Phenol has the lowest effective molality of particles and will have the highest freezing point. Thus, the order of the solutions in decreasing freezing point is: KBr > Glycerin > Phenol

Key concepts

Effective Molality

Effective molality is a key concept in understanding how solutions affect boiling and freezing points. It is related to colligative properties, which depend on the number of solute particles rather than their identity. Effective molality refers to the concentration of particles in a solution, considering how solutes dissociate or interact in the solvent.

• Definition: Effective molality is the total concentration of particles in a solution after considering dissociation or association. It is calculated by multiplying the molality of a solute by the number of particles it generates.
• Importance: Effective molality helps determine the extent to which freezing point depression and boiling point elevation will occur in a solution.

In the exercise, glycerin and phenol are molecular compounds, thus each yields only 1 particle per molecule when dissolved. KBr, an ionic compound, dissociates into two ions, doubling the particle count and hence the effective molality is greater. This shows why KBr has the most substantial effect on freezing point depression.

Aqueous Solutions

Aqueous solutions involve solutes dissolved in water, making water the solvent. This is a common scenario in chemistry because water is a universal solvent due to its polar nature, which allows it to dissolve many substances. Understanding aqueous solutions is crucial in both academic and real-world chemical applications.

• Aqueous Solution Properties: The properties of aqueous solutions, such as conductivity and boiling/freezing points, are affected by the nature and concentration of the dissolved solute.
• Role of Water: Water's ability to form hydrogen bonds is a significant factor in solute interactions, influencing the dissolution process and the resultant solution properties.

In the context of the exercise, the solutes glycerin, KBr, and phenol are all dissolved in water. The nature of each solute (ionic or molecular) alters the number of particles formed, which in turn affects the solution's freezing point.

Ionic and Molecular Compounds

Ionic and molecular compounds behave differently in solution, and this behavior affects properties like freezing point.

• Ionic Compounds: These consist of metals and nonmetals and dissociate into ions in solution. For example, KBr dissociates into K⁺ and Br^-. The dissociation results in a higher number of particles, increasing the effective molality.
• Molecular Compounds: These usually consist of nonmetals and do not dissociate into ions in water. Glycerin and phenol are examples, each forming one particle in solution. Consequently, their impact on colligative properties is generally less pronounced than ionic compounds.

Understanding whether a compound is ionic or molecular helps predict its effect on a solution's properties. In the exercise, recognizing that KBr is an ionic compound and glycerin and phenol are molecular helps explain the differences in freezing point depressions among the solutions.