13.7 osmotic pressure –osmosis, osmotic pressure calculations including molar mass determination,...

• 13.7 Osmotic Pressure – Osmosis, osmotic pressure calculations including molar mass

determination, colligative properties, practical applications (red blood cells, reverse osmosis, desalination of seawater)

• 13.8 Freezing-Point Depression and Boiling-Point Elevation of Nonelectrolyte Solutions

– Calculations involving molality of solute, freezing-point depression and boiling-point elevation, and the appropriate constants, molar mass and molecular formula determination, practical applications.

• 13.9 Solutions of Electrolytes – Van't Hoff factor and colligative properties, interionic attractions

June 17, 2009 – Class 45 and 46 Overview

• Osmosis: the net flow of solvent molecules through a semi-permeable membrane, from a more dilute solution (or from the pure solvent) into a more concentrated solution.

Osmotic Pressure

• Osmotic pressure (): the pressure that would have to be applied to a solution to stop the passage through a semi-permeable membrane of solvent molecules from the pure solvent.

• Osmotic pressure for dilute solutions of nonelectrolytes can be calculated by modifying the ideal gas equation:

Osmotic Pressure

nRTV

RTMRTV

n

Where = osmotic pressure, R = 0.08206 L atm mol-1 K-1, M = molarity

Problem: What is the osmotic pressure at 25 oC of an aqueous solution that is 0.0010 M C12H22O11 (sucrose)?

Problem: What mass of urea [CO(NH2)2] would you dissolve in 225 mL of solution to obtain an osmotic pressure of 0.015 atm at 25 oC?

Osmotic Pressure

Problem: Creatinine is a by-product of nitrogen metabolism and can be used to provide an indication of renal function. A 4.04 g sample of creatinine is dissolved in enough water to make 100.0 mL of solution. The osmotic pressure of the solution is 8.73 mmHg at 298 K. What is the molar mass of creatinine?

Problem: What would be the osmotic pressure of a solution containing 2.12 g of human serum albumin (a blood plasma protein; molar mass = 6.86 x 104 g/mol) in 75.00 mL of water at 37.0 oC?

Osmotic Pressure

Desalination of saltwater by reverse osmosis.

The membrane is permeable to water but not to ions.

The normal flow of water is from side A to side B.

If we exert a pressure on side B that exceeds the osmotic pressure of the saltwater, a net flow of water occurs in the reverse direction-from the saltwater to the pure water.

The lengths of the arrows suggest the magnitudes of the flow of water molecules in each direction

Osmotic Pressure – Practical Applications

Osmotic Pressure – Practical Applications

Suppose an animal or a plant cell is placed in a solution of sugar or salt in water:

Hypotonic: a dilute solution, with a higher water concentration than the cell; the cell will gain water through osmosis.

Isotonic: a solution with exactly the same water concentration as the cell; there will be no net movement of water across the cell membrane.

Hypertonic: a concentrated solution, with a lower water concentration than the cell; the cell will lose water by osmosis.

• Colligative properties: include vapor pressure lowering, freezing point depression, boiling point elevation and osmotic pressure. They have values that depend on the number of solute particles in a solution but not on their identity.

• Vapor pressure is lowered when a solute is present.– This results in boiling point elevation and freezing

point depression.

Colligative properties

Freezing-Point Depression:

Tf = freezing-point depression (oC)

(T – Tf, where T = freezing point of the solution, Tf = freezing point of the pure solvent)

Kf = proportionality constant (oC m-1)

(depends on melting point, enthalpy of fusion, and molar mass of the solvent)

m = molality = amount of solute (moles)

mass of solvent (kg)

Freezing-Point Depression and Boiling-Point Elevation of Nonelectrolyte Solutions

mKT ff

Boiling-Point Elevation:

Tb = boiling-point elevation (oC)

(T – Tb, where T = boiling point of the solution, Tb = boiling point of the pure solvent)

Kf = proportionality constant (oC m-1)

(depends on boiling point, enthalpy of vaporization, and molar mass of the solvent)

m = molality = amount of solute (moles)

mass of solvent (kg)


mKT bb

Problem: What is the molality of nicotine in an aqueous solution that starts to freeze at -0.450 oC? If this solution is obtained by dissolving 1.921 g of nicotine in 48.92 g of H2O, what must be the molar mass of nicotine?


• Van't Hoff factor (i): is a measure of the effect of a solute upon colligative properties, ie. it was observed that certain solutes produce a greater effect on colligative properties.

– Example: A 0.0100 m solution of NaCl in H2O has an experimental freezing point of -0.0361 oC.

• Note that this is consistent with the formation of 2 ions from the strong electrolyte, NaCl.

• For MgCl2, i = 3.

• For nonelectrolytes, i = 1• For weak electrolytes i has intermediate values. For example, for

acetic acid i is greater than 1, but significantly less than 2.

Solutions of Electrolytes

94.10100.086.1

0361.01

mCm

C

Tpredicted

Tmeasuredi

o

o

f

f

• Colligative property equations should now be re-written to account for the Van’t Hoff factor:

Solutions of Electrolytes

RTMiRTV

ni

mKiT ff

mKiT bb

Problem: Dr. Dawe’s mom adds salt to boiling water in order to cook spaghetti faster. How much salt needs to be added to 8 L of water at 1 atm in order to increase its boiling point by 1.5 oC? Assume the density of water to be 1 kg/L.

Problem: Predict the freezing point of aqueous 0.00145 m MgCl2.

Problem: What is the expected osmotic pressure of a 0.0530 M MgCl2 solution at 25 oC?


• Ions in solution do not behave independently of each other.

Interionic Attractions

The difference between the expected i and that observed is due to the electrostatic interactions of the ions and results in activities or effectiveconcentrations

Interionic Attractions

• Ions in solution do not behave independently of each other.

13.7 osmotic pressure –osmosis, osmotic pressure calculations including molar mass determination,...

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