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.)

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.)

Accepted Answer

Welcome back everyone. Lactic acid partially dissociates according to the equation where we have lactic acid in equilibrium with H plus a proton and our lactate anion as the second product, a solution is formed by dissolving 1.50 moles of lactic acid in one kg of water, where 1% of lactic acid dissociates to ions, calculate the freezing point of the solution given that the freezing point depression constant for water is 1.86 degrees Celsius per mole. So let's begin by calculating the morality. So that is lower case m the modality of lactic acid recall that we can determine that by calculating the moles of solute, our substance being dissolved divided by kilograms of solvent. So in this case, since we are told that we have 1% of lactic acid, which dissociates into ions. We would interpret that in total, we have 0.15 moles of protons plus the 0. moles of our lactate anion. And then added to that, we would have 1. moles of lactic acid that makes up our total moles of solute, which equates to the total of 1.50 moles of lactic acid in total. And then in our denominator, we would plug in the kilograms of solvent, which according to the prompt we're told is one point oh kilograms of water. This will simplify so that we have a modality for lactic acid equal to 1.515 mole. Now we're going to recall our freezing point depression formula where we would recall that we've got Delta T F for the change in freezing point depression equal to the modality of our substance multiplied by the freezing point depression, constant K F, plugging in our nodes to solve for the freezing point depression. Delta T F. We would have the modality of lactic acid 1.515 mole as we determined above which is multiplied by the freezing point constant of water. In this case, since we are given that in our prompt as 1.86 degrees Celsius per mola. And just to be clear that modality is the modality, not of just lactic acid, but of our solution just to make a correction. So the morality of our solution. And so we've got that multiplied by the freezing constant of our solvent water. And from this product, we would find the freezing point depression of 2.8179 degrees Celsius because the units of morality will cancel out. So now, in order to find the raising point of the solution, we would recall that that can be found by taking the freezing point depression of the solvent subtracted from the freezing point of our solvent. So we've got T F as the freezing point of the solvent minus delta T F, the freezing point depression of the solution. So, plugging in those values, we've got the freezing point of our solvent which in this case is water. And from our textbooks, we're called that is zero degrees Celsius that is then subtracted from the freezing point depression of the solution, which we calculated beforehand to be 2.8179 degrees Celsius. And this gives us a result of negative 2. degrees Celsius as the freezing point of the solution. And we're going to round this to about three significant figures to get a value of negative 2.82 degrees Celsius as the freezing point of the solution. And that would be our final answer to complete this example. I hope that this made sense and let us know if you have any questions.

Verified video answer for a similar problem:

Key Concepts

Dissociation of Acids

Colligative Properties

Freezing Point Depression Calculation