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Molality and Mole Fraction PowerPoint PPT Presentation


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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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Molality and Mole Fraction

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Molality and Mole Fraction

  • In Chapter 3 we introduced two important concentration units.

  • % by mass of solute

Molarity


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

Molality and Mole Fraction


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Molality and Mole Fraction

  • 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


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Molality and Mole Fraction

  • 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


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Molality and Mole Fraction

  • 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.


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Molality and Mole Fraction

  • The mole fraction of component B - XB


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Molality and Mole Fraction

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


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Colligative Properties of Solutions

  • 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.


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Lowering of Vapor Pressure and Raoult’s Law

  • 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.


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Lowering of Vapor Pressure and Raoult’s Law

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


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Fractional Distillation

  • 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.


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Boiling Point Elevation

  • 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.


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Boiling point elevation relationship is:

Boiling Point Elevation


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Freezing Point Depression

  • Relationship for freezing point depression is:


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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.


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Freezing Point Depression


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Boiling Point Elevation

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


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Freezing Point Depression

  • 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.


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Determination of Molecular Weight by Freezing Point Depression

  • 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.


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Determination of Molecular Weight by Freezing Point Depression

  • 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?


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Colligative Properties and Dissociation of Electrolytes

  • 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


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Osmotic Pressure

  • 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


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Osmotic Pressure

  • 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.


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Osmotic Pressure

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

    • M m


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Osmotic Pressure

  • 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


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Osmotic Pressure

  • 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.


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