Raoults Law, Nonvolatile Solute

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Raoults Law, Nonvolatile Solute

• Psoln Xsolvent Posolvent

Vapor Pressure of Solution
Vapor Pressure of Pure Solvent
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Raoults Law, Two Volatile Components

• Components A and B in an ideal binary solution
• Ptotal PA PB XA PoA XB PoB

Remember XA XB 1
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Two Volatile Components
Ideal solution A-A, B-B and A-B
interactions very similar (two molecules very
similar) ?Hsoln 0 Kind of like ideal gases.
Fig. 17.11 pg 863
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Nonideal Solutions
Negative Deviation from Raoults Law Solvent and
solute have high attraction ?Hsoln is large,
negative (ex. hydrogen bonding occurs) A-B
stronger than A-A and B-B Tendency of solvent
molecules to escape is lowered
Fig. 17.11 pg 863
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Nonideal Solutions
Positive Deviation from Raoults Law Solvent and
solute repel slightly ?Hsoln is positive A-B
weaker than A-A and B-B Increased tendency of
solvent molecules to escape
Fig. 17.11 pg 863
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Examples

• Negative Deviation
• Acetone and Water solution
• (attractive H-bonding)

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Example Question

• Would you expect an ethanol (CH3CH2OH) and
  methanol (CH3OH) mixture to be an ideal solution?
  
• No, hydrogen bonding interactions cause a
  negative deviation from Raoults law.

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Boiling Point Elevation

• A solution has a lower vapor pressure than the
  pure solvent (Psoln Xsolvent Posolvent)
• Therefore, a solution has a ________ boiling
  point than the pure solvent!

higher
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Boiling Point Elevation

• We previously calculated change in vapor pressure
  (Psoln Xsolvent Posolvent)
• Now we calculate change in boiling point

Table 17.5
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Example from Email

• What is the boiling point of a solution (1atm)
  that was prepared by dissolving 20.0g of urea in
  100.0g of water?
• urea MW 60.07 g/mol
• water Kb 0.51 C kg/mol

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Freezing Point Depression

• Solute molecules disturb solid-liquid equilibrium
  and lower the freezing point

Figure 17.13 pg 866
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Freezing Point Depression

• Adding salt to roads depresses the freezing
  point, helping turn ice into water

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Freezing Point Depression

• ?T is positive, even though temperature is lowered

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Example from Email

• What mass of glucose must be added to 1.0 L of
  water to produce a solution that freezes at -1.0
  C?
• glucose MW 180.2 g/mol
• water Kf 1.86 C kg/mol