Calculate the freezing point and the boiling point of each of the following solutions. (Assume c

Calculate the freezing point and the boiling point of each...

Key Concepts

Complete Dissociation

Complete dissociation assumes that an ionic compound fully separates into its constituent ions when dissolved in a solvent. This assumption simplifies colligative property calculations by allowing the use of the maximum possible van’t Hoff factor. It is a common approximation in introductory chemistry problems when dealing with strong electrolytes like salts.

van’t Hoff Factor

The van’t Hoff factor (i) is a measure of the number of particles into which a solute dissociates in solution. For ionic compounds, which dissociate into multiple ions, the factor is typically greater than one. This parameter is essential for accurately calculating the magnitude of colligative effects such as freezing point depression and boiling point elevation, especially when complete dissociation is assumed.

Boiling Point Elevation

Boiling point elevation is the effect observed when a non-volatile solute is added to a solvent, causing the boiling point of the resulting solution to be higher than that of the pure solvent. It is calculated using the equation ?Tb = i · Kb · m, where ?Tb is the increase in the boiling point, i is the van’t Hoff factor, Kb is the boiling point elevation constant, and m is the molality.

Colligative Properties

Colligative properties are solution properties that depend solely on the number of solute particles present in the solution, rather than on the identity of those particles. These properties include boiling point elevation and freezing point depression, and they arise because the addition of a solute affects the physical behavior of the solvent. This principle is central when calculating changes in boiling and freezing points upon solute addition.

Molality

Molality is a concentration unit defined as the number of moles of solute per kilogram of solvent. It is particularly useful in colligative properties calculations because it does not change with temperature and is directly involved in determining the extent of freezing point depression and boiling point elevation in solutions.

Freezing Point Depression

Freezing point depression refers to the phenomenon where the addition of a solute to a solvent lowers the temperature at which the liquid freezes compared to the pure solvent. This is quantitatively described by the equation ?Tf = i · Kf · m, where ?Tf is the decrease in the freezing point, i is the van’t Hoff factor, Kf is the freezing point depression constant of the solvent, and m is the molality.

Transcript

00:01 In part a, we are to calculate the freezing point depression and boiling point elevation for 5 grams of sodium chloride in 25 grams of water.

00:19 When sodium chloride is placed into water, it is considered an electrolyte and will ionize, which will yield two ions in aqueous solution.

00:34 We'll need that number later.

00:38 We'll then need to calculate the molality of the solution by first finding out the number of moles of nacl.

00:56 0 .855 moles.

01:01 Therefore, the molality is moles solutes divided by mass solvent, moles of solute, which is calculated.

01:22 The mass of the solvent, 25 grams of water, converted into kilograms.

01:35 Dividing will give us a molality of 3 .42 moles per kilogram or just lowercase m for units.

01:49 Change in freezing point for an electrolyte is the formula i constant, freezing point depression constant for water times a malality.

02:01 The i is the degree of ionization.

02:03 Sodium chloride and water produces two ions, n -a -plus and cl -minus, which we saw from the equation earlier.

02:11 The constant for water is 1 .86 times the molality, which will give us 12 .7 degrees celsius.

02:30 The freezing point of water is 0 degrees celsius minus the change 12 .7 degrees celsius for a freezing point.

02:42 Depression of negative 12 .7 degrees celsius...

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