Table of contents

1. Matter and Measurements4h 31m
2. Atoms and the Periodic Table5h 23m
3. Ionic Compounds2h 18m
4. Molecular Compounds2h 18m
5. Classification & Balancing of Chemical Reactions3h 17m
6. Chemical Reactions & Quantities2h 27m
7. Energy, Rate and Equilibrium3h 46m
8. Gases, Liquids and Solids3h 25m
9. Solutions4h 10m
10. Acids and Bases3h 29m
11. Nuclear Chemistry56m
BONUS: Lab Techniques and Procedures1h 38m
BONUS: Mathematical Operations and Functions47m
12. Introduction to Organic Chemistry1h 34m
13. Alkenes, Alkynes, and Aromatic Compounds2h 12m
14. Compounds with Oxygen or Sulfur1h 6m
15. Aldehydes and Ketones1h 1m
16. Carboxylic Acids and Their Derivatives1h 11m
17. Amines39m
18. Amino Acids and Proteins1h 51m
19. Enzymes1h 37m
20. Carbohydrates1h 41m
21. The Generation of Biochemical Energy2h 8m
22. Carbohydrate Metabolism2h 22m
23. Lipids2h 26m
24. Lipid Metabolism1h 45m
25. Protein and Amino Acid Metabolism1h 37m
26. Nucleic Acids and Protein Synthesis2h 54m

9. Solutions

Freezing Point Depression

GOB Chemistry

9. Solutions

Freezing Point Depression

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4:45 minutes

Problem 95b

Textbook Question

Many compounds are only partially dissociated into ions in aqueous solution. Trichloroacetic acid (CCl₃CO₂H), for instance, is partially dissociated in water according to the equation
CCl₃CO₂H(aq) → H⁺(aq) + CCl₃CO₂⁻ aq)
For a solution prepared by dissolving 1.00 mol of trichloroacetic acid in 1.00 kg of water, 36.0% of the trichloroacetic acid dissociates to form H⁺ and CCl₃CO₂⁻ ions.
b. What is the freezing point of this solution? (The freezing point of 1 kg of water is lowered 1.86 °C for each mole of solute particles.)

Verified step by step guidance

Step 1: Calculate the number of moles of trichloroacetic acid (CCl₃CO₂H) that dissociate. Since 36.0% of the acid dissociates, multiply the initial moles of trichloroacetic acid (1.00 mol) by 0.36 to find the moles of dissociated acid.

Step 2: Determine the total number of solute particles in the solution. For every mole of trichloroacetic acid that dissociates, it produces 2 particles (1 H⁺ ion and 1 CCl₃CO₂⁻ ion). Add the moles of dissociated particles to the moles of undissociated trichloroacetic acid (which remains as 1 particle per mole). The total number of solute particles is the sum of these contributions.

Step 3: Calculate the molality (m) of the solution. Molality is defined as the number of moles of solute particles per kilogram of solvent. Use the total number of solute particles from Step 2 and divide by the mass of the solvent (1.00 kg of water).

Step 4: Use the freezing point depression formula to calculate the change in freezing point (ΔT_f). The formula is ΔT_f = i × K_f × m, where i is the van 't Hoff factor (the number of particles per formula unit, which you calculated in Step 2), K_f is the freezing point depression constant for water (1.86 °C·kg/mol), and m is the molality from Step 3.

Step 5: Subtract the freezing point depression (ΔT_f) from the normal freezing point of water (0.00 °C) to find the freezing point of the solution.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Dissociation of Acids

Dissociation refers to the process by which an acid separates into its constituent ions in solution. In the case of trichloroacetic acid, it partially dissociates into hydrogen ions (H⁺) and trichloroacetate ions (CCl₃CO₂⁻). Understanding the extent of dissociation is crucial for calculating the concentration of ions in solution, which directly affects properties like freezing point.

Colligative Properties

Colligative properties are physical properties of solutions that depend on the number of solute particles rather than their identity. Freezing point depression is one such property, where the presence of solute particles lowers the freezing point of a solvent. The formula for freezing point depression indicates that for each mole of solute particles, the freezing point decreases by a specific amount, which is essential for solving the given problem.

Freezing Point Depression Calculation

To calculate the freezing point depression, one must determine the total number of solute particles in the solution. For trichloroacetic acid, 36.0% dissociation means that from 1.00 mol of acid, 0.36 mol dissociates into 0.36 mol of H⁺ and 0.36 mol of CCl₃CO₂⁻, resulting in a total of 0.72 mol of particles. This total is then used in the freezing point depression formula to find the new freezing point of the solution.

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