Using Freezing Point to determine Molar

MassObjectives:

To measure freezing points of known concentrations of solutionsTo use freezing point to determine

molar mass of compounds

Data

Soln FP ∆Tfp Molality

Molar Mass exp

Molar Mass theor.

% Error

Water

C6H12O

6

KNO3

Class Data

Soln FP FP FP FP FP FP

C6H12O

6

KNO3

The Set-up

Notes continued Solutions extend the liquid state of the

solvent ID of solute is not important. NUMBER

of particles in solution determines the effect

1 mole of particles reduces the freezing temp of 1 liter of water by 1.85°C = “molal freezing point constant” Kfp

Same type of relationship exists for BP m (molality) = mols solute/kg solvent m can be solved by this formula: m=

Tfp/Kfp

Examples of how Kfp is used

Soluble ionic solids dissociate C12H22O11

NaCl CaCl2 Kfp is per particle (moles of

particles), so must be multiplied by # of particles

Calculations

Use this equation to determine molar mass (exp):

g solute (1000 g) (1 kg) g solvent (1 kg) (mol solute) determine theoretical molar mass

Whiteboard

none

Freezing Point Lab Conclusion

1. On a particle level, why do you think FP is depressed in solutions?

2. If you tested for BP, why would you have to boil pure water first to determine water’s BP? Note, we assumed FP of water to be zero celsius.

3. CaCl2 is widely used to melt ice.a. Why does ice melt under the freezing point?b. Why is one mole of CaCl2 more effective than

one mole of NaCl?

Write Summary . . . .