Homework

# Colligative Properties and Colloids

## Chapter 13: 3-5 Homework

Textbook assignment: Read Kotz and Triechel, Chemistry and Chemical Reactivity Chapter 13: Sections 3 to 5.

##### Study Notes
1. 13.3 Solubility, or the amount of a specific solute that we can dissolve in a given solvent, depends upon pressure and temperature.
• Henry's Law shows that the solubility of a gas or liquid is directly proportional to gas pressure: Sg = kHPg, where k is the Henry's Law constant for the particular solute and solvent. Solubility increases if the gas pressure increases.
• Le Chatelier's principle describes how solubility changes with temperature. In general, a change in a factor determining equilibrium in a system will cause the system to adjust so that the effect of the change is counteracted. If we have an endothermic reaction (A + heat → B), and we add heat, the endothermic reaction will run forwards at an accelerated pace to reduce the amount of heat in the reactants. If we have an exothermic reaction (A → B + heat), and we had heat, the reaction will run backwards to reduce the amount of heat in the products.
2. 13.4 Colligative Properties are those properties which depend entirely on the ratio of solute particles to solvent particles.
• Raoult's Law describes the ideal behavior of the vapor pressure of a solvent in solution to the vapor pressure above a pure solvent: Psolvent = Xsolventsolvent where X is the mole fraction of the solvent in the solution. [Note that when we have a pure solution, X = 1, and the pressures are equal, as required.]
• As a result of Rauolt's law, the boiling point of a solvent will rise as it is diluted with solute, and its freezing point will drop. When ions are involved, the temperature change will be affected by a value (the van't Hoff factor, i).
• Boiling point elevation: ΔTbp = Kbpmsolute
• Freezing point depression: ΔTfp = Kfpmsolute
• Freezing point of ions: i = ΔTfp Measured/ΔTfp calculated = ΔT measured/Kfpm are a
• Osmotic pressure is the force exerted when a solution is separated by a semipermeable membrane into different concentrations. The laws of thermodynamics will drive the system to equilibrium, by forcing the solute to move from its high concentration area to the lower concentration area (diffusion), or that is not possible, by driving the solvent to move from the low concentration area to the high concentration area (osmosis). This pressure Π = cRT, where c is molarity, R is the gas constant, and T is the absolute temperature (in Kelvin). It's

Note: We can use a solute with a known K (determined experimentally under other circumstances) and measure the temperature change ΔTbp of a solution. From this data, we can solve for the molarity of the solute in the solution.

3. 13.5 Colloids or colloidal dispersions form when very large molecules can only diffuse slowly in a solvent. Such particles have high molecular masses and relatively large sizes, and they are big enough to scatter visible light, making the solutions in which they occur cloudy (Tyndall effect).
• A sol is a dispersion of a solid solute in a fluid medium (solvent).
• A gel is a dispersion that is immobile. Cooled jello is a gel.
• An emulsion is a collodial dispersion of one liquid in another, such as oil in water. a

#### Key Formula

ConceptFormulaNotes
Henry's Law Sg: gas solubility (mol/kg)
Pg: partial pressure of gas solute
kg: Henry's law constant for solute and solvent combination
Raoult's Law Psolvent: vapor pressure of solvent over solution
Xsolvent: mole fraction of solvent in solution
solvent: vapor pressure of solvent over pure solution
Boiling Point Elevation ΔTbp: Temperature increase
Kbp: molal boiling point elevation constant for solvent
msolute: moles solute
Freezing point depression ΔTfp: Temperature decrease
Kfp: molal freezing point depression constant for solvent
msolute: moles solute
Osmotic Pressure Π: osmotic pressure in atmospheres
R: gas constant
T: temperature (K)
van't Hoff factor ΔT: Temperature deviation from expected value
Kfp Freezing point depression constant
m: mass of solute
i: van't Hoff factor

#### Web Lecture

Read the following weblecture before chat: Solubility and Colligative properties

#### Videos for Chapter 13: Solutions and their Behavior

Review the Videos at Thinkwell Video Lessons.

• Under "Physical Properties of Solutions"
• Effects of Temperature and Pressure on Solubility
• Temperature Change and Solubility
• Extractions
• Pressure Change and Solubility
• Colligative Properties
• Vapor Pressure Lowering
• Boiling Point Elevation and Freezing Point Depression
• Osmosis
• Colligative Properties of Ionic Solutions
• Colloids
• Colloid Formation and Flocculation

#### Interactive exercise

Use the PhET States of Matter exercises to explore the molecular kinetics explanations for changes of phase.

#### Chat Preparation Activities

• Essay question: The Moodle forum for the session will assign a specific study question for you to prepare for chat. You need to read this question and post your answer before chat starts for this session.
• Mastery Exercise: The Moodle Mastery exercise for the chapter will contain sections related to our chat topic. Try to complete these before the chat starts, so that you can ask questions.

#### Chapter Quiz

• Required: Complete the Mastery exercise with a passing score of 85% or better.
• Go to the Moodle and take the quiz for this chapter.

#### (Aligns to) AP #10 GUIDED INQUIRY — Determination of solution concentration of copper sulfate solutions using visual colorimetry. — Phases II

Prepare a set of calibrated solutions, and compare transmittance, absorbance of light through the solutions as a function of concentration. Then attempt to estimate the concentration of an unknown solution by comparison with your calibrated solution.

• APGIE Lab 1: What is the Relationship Between the Concentration of a Solution and the Amount of Transmitted Light Through the Solution
• IGHCE Lab 7.5 Determine the Concentration of a Copper Sulfate Pentahydrate Solution through Visual Colorimetry
• HSCMK XIII-1: Determine Boron Concentration with Curcumin using Visual Colorimetry