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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
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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
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Ch.23 - Transition Metals and Coordination Compounds

Solution: What volume of ethanol (density = 0.7893 g/cm3) should be added to 450 mL of water in order to have a solution that freezes at -15.0°C? [For water, Kf = 1.86°C/m.]A) 371 mLB) 470 mLC) 212 mLD) 132 mLE) 167 mL
 

Solution: What volume of ethanol (density = 0.7893 g/cm3) should be added to 450 mL of water in order to have a solution that freezes at -15.0°C? [For water, Kf = 1.86°C/m.]A) 371 mLB) 470 mLC) 212 mLD) 132 mLE

Problem

What volume of ethanol (density = 0.7893 g/cm3) should be added to 450 mL of water in order to have a solution that freezes at -15.0°C? [For water, Kf = 1.86°C/m.]

A) 371 mL

B) 470 mL

C) 212 mL

D) 132 mL

E) 167 mL


 

Solution

We’re being asked to determine the volume of ethanol (C2H5OH) that must be added to 450 mL water to get a solution with a freezing point of –15.0 ˚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: Calculate for ΔTf.

Step 2: Determine the molality of the solution.

Step 3: Calculate the volume of C2H5OH needed.


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