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# Molality and Mole Fraction - PowerPoint PPT Presentation

Molality and Mole Fraction. In Chapter 3 we introduced two important concentration units. % by mass of solute. Molarity. Molality is a concentration unit based on the number of moles of solute per kilogram of solvent . Molality and Mole Fraction. Molality and Mole Fraction.

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Presentation Transcript

• In Chapter 3 we introduced two important concentration units.

• % by mass of solute

Molarity

Molality is a concentration unit based on the number of moles of solute per kilogram of solvent.

Molality and Mole Fraction

Molality and Mole Fraction moles of solute per

• Calculate the molarity and the molality of an aqueous solution that is 10.0% glucose, C6H12O6. The density of the solution is 1.04 g/mL. 10.0% glucose solution has several medical uses. 1 mol C6H12O6 = 180 g

Molality and Mole Fraction moles of solute per

• Calculate the molality of a solution that contains 7.25 g of benzoic acid C6H5COOH, in 2.00 x 102 mL of benzene, C6H6. The density of benzene is 0.879 g/mL. 1 mol C6H5COOH = 122 g

Molality and Mole Fraction moles of solute per

• Mole fraction is the number of moles of one component divided by the moles of all the components of the solution

• Mole fraction is literally a fraction using moles of one component as the numerator and moles of all the components as the denominator.

• In a two component solution, the mole fraction of one component, A, has the symbol XA.

Molality and Mole Fraction moles of solute per

• The mole fraction of component B - XB

Molality and Mole Fraction moles of solute per

• What are the mole fractions of glucose and water in a 10.0% glucose solution?

Colligative Properties of Solutions moles of solute per

• Colligative properties are properties of solutions that depend solely on the number of particles dissolved in the solution.

• Colligative properties do not depend on the kinds of particles dissolved.

• Colligative properties are a physical property of solutions.

• There are four common types of colligative properties:

• Vapor pressure lowering

• Freezing point depression

• Boiling point elevation

• Osmotic pressure

• Vapor pressure lowering is the key to all four of the colligative properties.

Lowering of Vapor Pressure and Raoult’s Law moles of solute per

• Addition of a nonvolatile solute to a solution lowers the vapor pressure of the solution.

• The effect is simply due to fewer solvent molecules at the solution’s surface.

• The solute molecules occupy some of the spaces that would normally be occupied by solvent.

• Raoult’s Law models this effect in ideal solutions.

Lowering of Vapor Pressure and Raoult’s Law moles of solute per

• This graph shows how the solution’s vapor pressure is changed by the mole fraction of the solute, which is Raoult’s law.

Fractional Distillation moles of solute per

• Distillation is a technique used to separate solutions that have two or more volatile components with differing boiling points.

• A simple distillation has a single distilling column.

• Simple distillations give reasonable separations.

• A fractional distillation gives increased separations because of the increased surface area.

• Commonly, glass beads or steel wool are inserted into the distilling column.

Boiling Point Elevation moles of solute per

• Addition of a nonvolatile solute to a solution raises the boiling point of the solution above that of the pure solvent.

• This effect is because the solution’s vapor pressure is lowered as described by Raoult’s law.

• The solution’s temperature must be raised to make the solution’s vapor pressure equal to the atmospheric pressure.

• The amount that the temperature is elevated is determined by the number of moles of solute dissolved in the solution.

Boiling point elevation relationship is: moles of solute per

Boiling Point Elevation

Freezing Point Depression moles of solute per

Fundamentally, freezing point depression and boiling point elevation are the same phenomenon.

The only differences are the size of the effect which is reflected in the sizes of the constants, Kf & Kb.

This is easily seen on a phase diagram for a solution.

Freezing Point Depression

• Notice the similarity of the two relationships for freezing point depression and boiling point elevation.

Freezing Point Depression elevation are the same phenomenon.

Boiling Point Elevation elevation are the same phenomenon.

• What is the normal boiling point of a 2.50 m glucose, C6H12O6, solution?

Freezing Point Depression elevation are the same phenomenon.

• Calculate the freezing point of a solution that contains 8.50 g of benzoic acid (C6H5COOH, MW = 122) in 75.0 g of benzene, C6H6.

• The size of the freezing point depression depends on two things:

• The size of the Kf for a given solvent, which are well known.

• And the molal concentration of the solution which depends on the number of moles of solute and the kg of solvent.

• If Kf and kg of solvent are known, as is often the case in an experiment, then we can determine # of moles of solute and use it to determine the molecular weight.

• A 37.0 g sample of a new covalent compound, a nonelectrolyte, was dissolved in 2.00 x 102 g of water. The resulting solution froze at -5.58oC. What is the molecular weight of the compound?

• Electrolytes have larger effects on boiling point elevation and freezing point depression than nonelectrolytes.

• This is because the number of particles released in solution is greater for electrolytes

• One mole of sugar dissolves in water to produce one mole of aqueous sugar molecules.

• One mole of NaCl dissolves in water to produce two moles of aqueous ions:

• 1 mole of Na+ and 1 mole of Cl- ions

Osmotic Pressure Depression

• Osmosis is the net flow of a solvent between two solutions separated by a semipermeable membrane.

• The solvent passes from the lower concentration solution into the higher concentration solution.

• Examples of semipermeable membranes include:

• cellophane and saran wrap

• skin

• cell membranes

Osmotic Pressure Depression

• Osmosis is a rate controlled phenomenon.

• The solvent is passing from the dilute solution into the concentrated solution at a faster rate than in opposite direction, i.e. establishing an equilibrium.

• The osmotic pressure is the pressure exerted by a column of the solvent in an osmosis experiment.

Osmotic Pressure Depression

• For very dilute aqueous solutions, molarity and molality are nearly equal.

• M m

Osmotic Pressure Depression

• Osmotic pressures can be very large.

• For example, a 1 M sugar solution has an osmotic pressure of 22.4 atm or 330 p.s.i.

• Since this is a large effect, the osmotic pressure measurements can be used to determine the molar masses of very large molecules such as:

• Polymers

• Biomolecules like

• proteins

• ribonucleotides

Osmotic Pressure Depression

• A 1.00 g sample of a biological material was dissolved in enough water to give 1.00 x 102 mL of solution. The osmotic pressure of the solution was 2.80 torr at 25oC. Calculate the molarity and approximate molecular weight of the material.