A student dissolves 45.0 g of an unknown solid in 225.0 g of cyclohexane. It is known to disso

step 1 So the cyclohexane freezes at 6 .47. So the difference in the freezing point will be 6 .47 minus

A student dissolves 45.0 g of an unknown solid in 225.0 g...

A student dissolves $45.0 \mathrm{~g}$ of an unknown solid in $225.0 \mathrm{~g}$ of cyclohexane. It is known to dissolve without dissociating. She cools the solution and finds that the temperature remains at $2.70^{\circ} \mathrm{C}$ while the solution changes phase from liquid to solid. What is the molar mass of the solid?

Key Concepts

Colligative Properties

Colligative properties are characteristics of solutions that depend only on the number of solute particles present in the solvent and not on their chemical identity. These properties, including boiling point elevation, freezing point depression, vapor pressure lowering, and osmotic pressure, are useful for determining properties such as molar mass because they relate the changes observed in a solvent to the concentration of the solute.

Freezing Point Depression

Freezing point depression is a colligative property where the addition of a solute to a solvent lowers the temperature at which the solvent freezes. The extent of this depression (?Tf) can be calculated using the equation ?Tf = Kf × m, where Kf is the freezing point depression constant specific to the solvent and m is the molality of the solution. This property allows one to relate the amount of solute dissolved to the change in the freezing point of the solution.

Molality

Molality is a concentration unit defined as the number of moles of solute per kilogram of solvent. It is particularly useful in calculations involving colligative properties because it remains unaffected by temperature changes, unlike molarity. In freezing point depression problems, molality is used to quantify how concentrated a solution is, which in turn helps determine the number of solute particles present.

Molar Mass

Molar mass is the mass of one mole of a substance, typically expressed in grams per mole. Determining the molar mass of an unknown substance can involve measuring the number of moles required to produce a certain colligative effect, such as a drop in freezing point. By relating the measured mass of the solute to the calculated number of moles (using the freezing point depression data), the molar mass can be determined.

Transcript

Please give Ace some feedback