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Ch.9 Solutions
McMurry - Fundamentals of General, Organic, and Biological Chemistry 8th Edition
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Capitolo 9, Problema 95a

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

Guida verificata passo dopo passo
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Step 1: Understand the dissociation process. Trichloroacetic acid (CCl₃CO₂H) is a weak acid that partially dissociates in water. The dissociation reaction is: CCl₃CO₂H (aq) → H⁺ (aq) + CCl₃CO₂⁻ (aq). This means that for every mole of CCl₃CO₂H that dissociates, 1 mole of H⁺ and 1 mole of CCl₃CO₂⁻ are produced.
Step 2: Calculate the number of moles of trichloroacetic acid that dissociate. Since 36.0% of the acid dissociates, multiply the initial moles of CCl₃CO₂H (1.00 mol) by 0.36 to find the moles of dissociated acid.
Step 3: Determine the moles of ions produced. For every mole of dissociated CCl₃CO₂H, 1 mole of H⁺ and 1 mole of CCl₃CO₂⁻ are formed. Therefore, the moles of H⁺ and CCl₃CO₂⁻ ions will each equal the moles of dissociated CCl₃CO₂H. Add these two values together to find the total moles of ions produced.
Step 4: Calculate the moles of undissociated CCl₃CO₂H remaining. Subtract the moles of dissociated CCl₃CO₂H (calculated in Step 2) from the initial moles of CCl₃CO₂H (1.00 mol). This gives the moles of undissociated molecules.
Step 5: Add the moles of undissociated CCl₃CO₂H to the total moles of ions (from Step 3) to find the total concentration of dissolved species (ions and molecules) in the solution. Since the solution is prepared in 1.00 kg of water, the total moles calculated represent the total concentration in mol/kg.

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Dissociation of Acids

Dissociation refers to the process by which an acid separates into its constituent ions when dissolved in water. 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. In this context, it involves calculating the total concentration of all species present, including both the undissociated acid and the ions produced from dissociation. This requires using the initial amount of acid and the percentage that dissociates to find the final concentrations.
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Molarity and Total Ion Concentration

Molarity is defined as the number of moles of solute per liter of solution. To find the total concentration of dissolved ions and molecules, one must consider both the dissociated ions and the undissociated acid. This involves summing the concentrations of H⁺ ions, CCl₃CO₂⁻ ions, and the remaining undissociated trichloroacetic acid in the solution.
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