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Ch.9 Solutions
Chapter 9, Problem 95

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. What is the total concentration of dissolved ions and molecules in 1 kg of water?

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Identify the initial amount of trichloroacetic acid: 1.00 mol.
Calculate the amount of trichloroacetic acid that dissociates: 36.0% of 1.00 mol.
Determine the moles of H⁺ and CCl₃CO₂⁻ ions formed, which is equal to the moles of dissociated trichloroacetic acid.
Calculate the moles of undissociated trichloroacetic acid remaining: 1.00 mol - moles dissociated.
Add the moles of undissociated trichloroacetic acid and the moles of ions to find the total concentration of dissolved species.

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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. For weak acids like trichloroacetic acid, this process is not complete, meaning only a fraction of the acid molecules release protons (H⁺) and form anions. Understanding the degree of dissociation is crucial for calculating the concentrations of ions in solution.
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Concentration Calculations

Concentration is a measure of the amount of a substance in a given volume of solution, typically expressed in moles per liter (M). To find the total concentration of dissolved species, one must account for both the undissociated acid and the ions produced from dissociation. This involves using the initial amount of acid and the percentage that dissociates to determine the final concentrations.
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Molarity and Total Ion Concentration

Molarity (M) is defined as the number of moles of solute per liter of solution. In this context, to find the total concentration of dissolved ions and molecules, one must consider both the concentration of the dissociated ions and the remaining undissociated acid. The total concentration is the sum of the concentrations of all species present in the solution, providing a comprehensive view of the solution's composition.
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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. 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.)
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Hyperbaric chambers, which provide high pressures (up to 6 atm) of either air or pure oxygen, are used to treat a variety of conditions, ranging from decompression sickness in deep-sea divers to carbon monoxide poisoning. Look up the solubility of O₂, N₂, CO, and CO₂ in water at standard temperature and pressure (1 atm, 25 °C). Explain the trends in relative solubility for these gases. (Refer to Section 8.2 and Section 9.2)
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