Colligative Properties of Solutions Sec 14-2
Welcome message from author
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Colligative Properties of Solutions
Sec 14-2
Colligative Properties
What are they? Properties that depend on the concentration of
solute particles but not on their identity.
For calculations, we must use concentrations in molality.
Molality Review
Molality = moles solute
mass of solvent (kg)
Ex) A solution was prepared by dissolving 17.1 g of sucrose (MW 342.34g/mol) in 125 g of water. Calc it molality.
17.1 g x 1mol/342.34g = 0.0500 mol sucrose
m = 0.0500mol/ 0.125kg = 0.400 m solu.
Vapor Pressure Lowering
The boiling point and freezing point of a solution differ from those of a pure solvent.
A non-volatile solute will increase boiling point and decrease freezing point.
Vapor Pressure Lowering
Works by reducing the vapor pressure. Vapor pressure is lower because the solute takes
up space at the surface of the liquid.
Freezing-Point Depression
The freezing point of a 1molal solution of any nonelectrolyte solute in water will decrease the freezing point by 1.86o C. Molal freezing point Constant (Kf): = the freezing
point depression of the solvent in a 1 molal solution of a nonvolatile, nonelectrolyte solute. Kf of water: = -1.86o C
Freezing-Point Depression
Each solvent has its own characteristic molal freezing point. (see table 14-2 pg 438) These values are most accurate for dilute
solutions at 1 atmosphere pressure.
Freezing-Point Depression
Freezing Point Depression (Δtf) is the difference between the freezing point of the pure solvent and a solution of a nonelectrolyte in that solvent. is directly proportional to the molal concentration
of the solution Δtf = freezing point (f.p.)solution – f.p. pure solvent
Δtf = Kfm Kf = molal freezing point constantm = molal concentration
Example problem freezing point depression What is the freezing-point depression of
water in a solution of 17.1g sucrose (MW 342.34g/mol) and 200.g of water. What is the actual freezing point of the solution?
1st calculate the number of moles of solute
2nd determine the molal concentration of solu.
3rd. Determine Δtf
Example freezing point depression A water solution containing an unknown
quantity of a nonelectrolyte solute is found to have a freezing point of -0.23oC. What is the molal concentration of the solution? Know
Water is solvent so Kf = -1.86oC/m
Δtf = f.p. solution – f.p. pure solvent
= -0.23oC - 0.00oC = -0.23oC
Rearrange Δtf = Kfm to solve for m m= Δtf / Kf
Boiling Point Elevation
Molal boiling point constant (Kb) is the boiling point elevation of the solvent in a 1 molal solution of a nonvolatile, nonelectrolyte solute. For water Kb = 0.51oC/m
Boiling point elevation (Δtb) is the difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent. Δtb=Kbm Δtb= b.p. solu – b.p. solvent
Example problem boiling point elevation What is the boiling point elevation of a
solution made from 20.0g of a nonelectrolyte solute and 400.0g of water? (MW = 62.0g/mol)
Electrolytes and Colligative Properties Early experiments showed that some
solutions depressed the freezing point of a solvent more than expected.
Ex) A 0.1m solution of NaCl lowers the f.p. of a
solvent nearly twice as much as a 0.1m solution of sucrose. And 0.1m solution of CaCl2 lowered the f.p. nearly three times as much as a 0.1m solution of sucrose.
Electrolytes and Colligative Properties WHY?
Compare the behavior of sucrose with that of NaCl in aqueous solution Sucrose dissolves to yield only 1 mole of particles is
solution since it is a nonelectrolyte NaCl dissolves to yield 2 moles of particles in solution
since it is a strong electrolyte
1mol NaCl 1mol Na+ + 1mol Cl-
Electrolytes and Colligative Properties
Calculated Values for Electrolyte Solutions For electrolytes we must figure molality in
terms of total moles of dissolved particles rather than formula units.
Ex) molality of solution x molality conversion = adjusted molality
22.99 g of NaCl
Calculated Values for Electrolyte Solutions Example problem What is the expected change in the freezing
point of water in a solution of 62.5 g of barium nitrate, Ba(NO3)2 in 1.00 kg of water
-1.33oC
Calculated Values for Electrolyte Solutions Example problem What is the expected freezing point
depression for a solution that contains 2.0 moles of magnesium sulfate dissolved in 1.0 kg of water?
-7.4oC
Related Documents
