1. Boiling and Freezing Points
Chapter 21
Boiling and Freezing Points

2. Boiling & Freezing Pts.
Presence of nonvolatile solute particles causes boiling pt. of soln. to be raised.
Vapor pressure is lowered, \ need a higher temp. to get vapor press. of liquid to = atmos. press.
Freezing Pt. - which vapor press of solid & liquid are =
Since addition of solute particles lower vapor press, solns. lower temp.

3. Boiling & Freezing Pts.
\ The addition of a nonvolatile solute to a liquid causes boiling pt. elevation & freezing pt. depression
Occurs bec. of vapor press. of solvent being lowered by solute
Change depends on concen. of solute & not its chemical properties.

4. Calculating Boiling & Freezing Pts.
1 mole of nonvolatile solute particles will raise the boiling temp. of 1 kg of water oC (molal boiling pt. elevation constant for water)
The same conc. will lower the freezing pt. of 1 kg of water oC (molal freezing pt. constant for water)

5. Calculating Boiling & Freezing Pts.
A 1 m soln. of sugar contains 1 mole of solute particles per kg water
A 1 m soln. of NaCl contains 2 moles of solute particles per kg water
1 mole Na+ & 1 mole Cl-
A 1 m soln. of CaCl2 contains 3 moles of solute particles per kg water
1 mole Ca+2 & 2 moles Cl-

6. Calculating Boiling & Freezing Pts.
1 m NaCl soln. freezes ~ 2(1.853) below normal freezing pt. of water
1 m CaCl2 freezes ~ 3(1.853) below normal
\ to determine change in freezing pt. or boiling pt., multiply the freezing (or boiling) pt. constant by molality in total particles
Dfp = m (Kfp) and Dbp = m (Kbp)
Then add or subtract from normal f.p or b.p.

7. Calculating Boiling & Freezing Pts.
If 85.0 grams of sugar are dissolved in 392 g of water , what will be the boiling point and freezing point of the resulting solution?

8. Calculating Boiling & Freezing Pts.
If 26.4 g of Nickel (II) bromide are dissolved in 224 g of water, what will be the boiling point and freezing point of the resulting soln.?

9. Experimental Determination of Molecular Mass
Molec. Mass of a solute may be found by using b.p. elevation or f.p. depression if a known mass of solute is added to a known mass of solvent and the resulting shift in b.p. or f.p. is measured.

10. Experimental Determination of Molecular Mass
DTbp = m Kbp
\ m = DTbp
Kbp
Once molality is found, molec. mass (g/mol) can be found mathematically.

11. Experimental Determination of Molecular Mass
If 99.0 g of nonionizing solute are dissolved in 669 g of water, and the freezing point of the resulting soln. is oC, what it the molecular mass of the solute?