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Section 16.4 Lecture for Honors Chem
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Chemistry 16.4Bellwork- Colligative Fill-inColligative properties depend only on the _________of solute particles and not on the __________ ofsolute particles.
A mole of ionic solute will produce a _______change in colligative properties than a mole ofmolecular (covalent) solute because it will produce_________ particles.
Colligative properties include vapor pressure________, freezing point ________, and boiling point________.
Fill- in the blank exercises are __________.
Calculations Involving Colligative Properties
Cooking instructionsoften call for the additionof a small amount of saltto the cooking water.Dissolved salt elevatesthe boiling point ofwater. You will learn howto calculate the amountthe boiling point of thecooking water rises.
The unit molality and mole fractions aretwo additional ways in which chemistsexpress the concentration of a solution.
16.4
The unit molality (m) is the number ofmoles of solute dissolved in 1 kilogram(1000 g) of solvent.
Molality is also known as molalconcentration.
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To make a 0.500msolution of NaCl, usea balance tomeasure 1.000 kg ofwater and add 0.500mol (29.3 g) of NaCl.
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for Sample Problem 16.6
The mole fraction of a solute ismoles of solute divided bytotal moles (moles solute + moles solvent).
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nA = moles of A
16.7
Ethlylene Glycol (EG) is added to wateras antifreeze.
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Sample Problem 16.716.7
for Practice Problem 16.7
The freezing-point depression (∆Tf) and theboiling-point elevation (∆Tb) of a solution aredirectly proportional to the molalconcentration (m) of solute particles.
16.4
∆Tf = i Kf m
Kf is a constant, the molal freezing-point depression constant, which isdifferent for every solvent.
i is the number of particles the solutemakes.
NaCl i = 2 MgCl2 i = 3
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∆Tb = i Kb m
The constant, Kb, is the molal boiling-point elevation constant, which is equalto the change in boiling point for a 1-molal solution of particles.
1m MgCl2 = 3m particles
1m sugar = 1m particles
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for Sample Problem 16.8
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for Sample Problem 16.9
16.4 Section Quiz.
1. What is the mole fraction of He in a gaseoussolution containing 4.0 g of He, 6.5 g of Ar,and 10.0 g of Ne?
a. 0.60
b. 1.5
c. 0.20
d. 0.11
16.4 Section Quiz.
2. The freezing point depression caused by agiven concentration of a nonvolatile molecularsolute
a. depends on the solute.
b. depends on the solvent.
c. is always the same.
d. cannot be determined.
16.4 Section Quiz.
3. What are the freezing and boiling points of a0.1m solution of CaCl2 in water?
a. -0.2°C, 100.1°C
b. -0.6°C, 100.1°C
c. -0.6°C, 100.2°C
d. -0.6°C, 99.8°C
16.4 Section Quiz.
4. Compared to the freezing point depression byethylene glycol (C2H6O2,) for a given solvent,the freezing point depression caused by thesame molal concentration of CaCl2 would be
a. exactly the same.
b. twice as large.
c. three times as large.
d. four times as large
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