The normal boiling point of ethanol, CH3 CH2 OH, is 78.4^∘ C. When 3.26 g of a soluble nonelect

step 1 In this problem, we are to consider a solution in ethanol. The normal boiling point of ethanol i

The normal boiling point of ethanol, CH3 CH2 OH, is 78.4^∘...

Key Concepts

Colligative Properties

Colligative properties are characteristics of solutions that depend solely on the number of solute particles present, rather than their chemical identity. This means that when a nonvolatile solute is dissolved in a solvent, properties such as boiling point elevation, freezing point depression, vapor pressure lowering, and osmotic pressure change in a way that is determined only by the concentration of the solute particles compared to the solvent molecules.

Vapor Pressure Lowering

Vapor pressure lowering is a colligative property that occurs when a nonvolatile solute is added to a volatile solvent. The presence of solute particles decreases the mole fraction of the solvent and, according to Raoult’s law, this reduction in mole fraction results in a corresponding reduction in the vapor pressure of the solvent. This phenomenon underlies many other observed effects, such as boiling point elevation and freezing point depression.

Raoult’s Law

Raoult’s law provides a quantitative relationship for the vapor pressure of a solution by stating that the partial vapor pressure of a solvent is equal to the mole fraction of the solvent multiplied by the vapor pressure of the pure solvent. It is fundamental for calculating how the addition of a solute, particularly a nonvolatile one, alters the vapor pressure and hence the phase change temperatures of the solvent.

Boiling Point Elevation

Boiling point elevation is the phenomenon where the boiling point of a solvent increases when a nonvolatile solute is dissolved in it. This increase occurs because the addition of the solute lowers the vapor pressure of the solvent, requiring a higher temperature to reach the atmospheric pressure at which boiling occurs. The quantitative equation ?Tb = Kb · m links the elevation to the molal concentration of the solute, where Kb is the ebullioscopic constant of the solvent.

Molar Mass Determination Using Colligative Properties

Molar mass determination using colligative properties involves measuring changes in properties such as boiling point or freezing point when a solute is dissolved in a solvent. By comparing the measured shift (for example, boiling point elevation) with theoretical predictions based on the solute’s molality, and using relationships like ?Tb = Kb · m, one can determine the number of moles of solute present. With the known mass of the solute, this information can then be used to calculate its molar mass.

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