Review To join clicker to class today: – Turn on the Clicker (the red LED comes on). – Push...

ReviewTo join clicker to class today:

– Turn on the Clicker (the red LED comes on).

– Push “Join” button followed by “20” followed by the “Send” button (switches to flashing green LED if successful).

● Water solutions of non-electrolytes (molecular compounds) do not conduct electricity.

● Colligative properties. ● Example 1: Osmosis

− Osmotic pressure ∏= MRT− Calculated pressure of 0.50 M sugar relative to DI water. ∏=(0.50 mol/L)(0.08206 atm•L•mol-1 •K-1)(298 K) = 12. atm (1.2 x 101 atm)

• Conversion between molarity and (mass solute)/(mass sol'n).

● Water solutions of electrolytes conduct electricity.

Boiling Point Elevation• Solute in a liquid lowers the rate at which

molecules can enter the gas phase by sticking to solvent molecules or blocking access to surface, raising the boiling point.

• The observed effect is again proportional to the solute particle concentration: ∆Tb = imKb.– i = the van’t Hoff factor– m = molality = (moles solute/kg solvent)

– Kb = the boiling point elevation constant which depends on the identity of the solvent.

Molality (m)• Molality = (moles solute)/(kg of solvent)

NOTE DENOMINATOR IS NOT kg of solution!

• Ex. 0.50 moles NaCl in 2.0 kg water. m = 0.50 moles NaCl/(2.0 kg solvent) = 0.25 m NaCl in water.

• Compare with molarity (M). What is the molality of 2.00 M NaCl in water ?– assume solution density =1.03 g/mL=>1.03 kg

sol’n/L sol’n, – MM(NaCl) = 58.443 g/mole

Boiling Point Elevation Calculation.

• Relation: ∆Tb = imKb

• Ex. How much higher is Bp of 0.500 m NaCl than pure water? – Kb(H2O) = 0.5121 K or ˚C/m

– Assume i = ?

Freezing Point Depression• Same idea as Bp elevation except that solute gets in

the way of solid formation and lowers the freezing point.

• Observed relation is: ∆Tf = imKf

– Note that Kf is usually reported as positive

– ∆T comes out positive. – You must subtract a positive ∆T from the freezing point of

the pure solvent since the freezing point drops.

● Other types of calculations you could do. − How much CaCl

2 do you need in some amount of water to

drop the freezing point 10 ˚C? − What morality CaCl

2 sol'n is necessary?

− Account for the van't Hoff factor.