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Ch.1 - Intro to General Chemistry
Ch.2 - Atoms & Elements
Ch.3 - Chemical Reactions
BONUS: Lab Techniques and Procedures
BONUS: Mathematical Operations and Functions
Ch.4 - Chemical Quantities & Aqueous Reactions
Ch.5 - Gases
Ch.6 - Thermochemistry
Ch.7 - Quantum Mechanics
Ch.8 - Periodic Properties of the Elements
Ch.9 - Bonding & Molecular Structure
Ch.10 - Molecular Shapes & Valence Bond Theory
Ch.11 - Liquids, Solids & Intermolecular Forces
Ch.12 - Solutions
Ch.13 - Chemical Kinetics
Ch.14 - Chemical Equilibrium
Ch.15 - Acid and Base Equilibrium
Ch.16 - Aqueous Equilibrium
Ch. 17 - Chemical Thermodynamics
Ch.18 - Electrochemistry
Ch.19 - Nuclear Chemistry
Ch.20 - Organic Chemistry
Ch.22 - Chemistry of the Nonmetals
Ch.23 - Transition Metals and Coordination Compounds

Solution: When 0.81 g of an unknown non-electrolyte is dissolved in 50.0 g of benzene, the freezing point decreased by 0.41 degrees C.If the Kfp of the solvent is 5.12 K/m, calculate the molar mass of the unknown solute.a. 20.23 g/mol b. 202.3 g/mol c. 2023 g/mol d. not enough information is given

Solution: When 0.81 g of an unknown non-electrolyte is dissolved in 50.0 g of benzene, the freezing point decreased by 0.41 degrees C.If the Kfp of the solvent is 5.12 K/m, calculate the molar mass of the unkno

Problem

When 0.81 g of an unknown non-electrolyte is dissolved in 50.0 g of benzene, the freezing point decreased by 0.41 degrees C.If the Kfp of the solvent is 5.12 K/m, calculate the molar mass of the unknown solute.

a. 20.23 g/mol
b. 202.3 g/mol
c. 2023 g/mol
d. not enough information is given

Solution

We’re being asked to determine the molar mass of an unknown solute present in 50.0 g benzene to get a change in freezing point of 0.41˚C.


Recall that the freezing point of a solution is lower than that of the pure solvent and the change in freezing point (ΔT­f) is given by:


ΔTf=Tf, pure solvent-Tf, solution


The change in freezing point is also related to the molality of the solution:


ΔTf=imKf

where: 

i = van’t Hoff factor

m = molality of the solution (in m or mol/kg)

Kf = freezing point depression constant (in ˚C/m)


Recall that the molality of a solution is given by:


molality=moles solutekg solvent


For this problem, we need to do the following:

Step 1: Determine the molality of the solution.

Step 2: Calculate the moles of the unknown solute.

Step 3: Calculate the molar mass of the unknown solute.


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