Freezing Point Depression

Purpose of the Freezing Point Depression Lab

To construct cooling curves of pure and mixed lauric acid to determine the freezing-point depression and the molar mass of an unknown organic substance.

Colligative Properties

Properties that only depend on the number of solute particles in

solution and not, the identity of the solute.

What happens to vapor pressure when solute is added?

Vapor pressure lowers when solute is added

The solute particles stop the solvent

particles in transitioning to the vapor phase

What happens to boiling point when solute is added?

Boiling point increases when solute is added

What happens to freezing point depression when solute is added

Freezing point depression lowers when solute is added

List the 4 colligative properties

1. Vapor pressure

2. Boiling point

3. Freezing point

4. Osmotic pressure

Boiling Point Elevation Graph

Solid to the left

Liquid in the Y

Vapor to the right

Dashed Line = presence of solute

Freezing-Point Depression Equation

ΔTf = iKfm

What is ΔTf in the freezing point depression equation?

ΔTf = Tºf – T f

Tºf = freezing point of pure solvent

T f = freezing point of the solution

What is Kf in the freezing point depression equation?

The molal freezing point depression constant (0C/m) that is specific to the solvent

what does m stand for in the freezing point depression equation?

m = molality, moles of solute/ kg of solvent

What data do you need to determine molar mass using freezing point depression?

• Amount of solvent

• Amount of solute added

• Freezing point (or b.p.) before and after adding solute

What is the Van’t Hoff Factor (i) the freezing point depression equation?

Number of particles a solute forms in solution

( i = 1 for nonionic compounds)

Whats the purpose of acetone in this lab?

To remove lauric acid residue

What are the hazards for this lab?

Lauric acid, and the unknown solids are all Flammable.

They are all eye, skin and respiratory irritants.

Calculation Example

A solution containing 0.050 g of an unknown nonelectrolyte in 2.50 g of Cyclohexane was found to freeze at 5.1ﹾC . What is the molar mass of the Unknown substance ?

Kf of cyclohexane = 20.4 ﹾC/m

Freezing point of pure cyclohexane = 6.6 ﹾC

1. find change of temp: ΔT = 6.6 – 5.1 = 1.5 ﹾC

2. Substitute in thethe equation to find molality: ΔT = Kf m

1.5 ﹾC = 20.4 ﹾC/m x m

m = 1.5 / 20.4 = 0.0735 = moles of solute

3. Divide g of solute by 1000 kg 2.50 g /1000 = 0.0025 kg

4. Divide moles of solute by kg = 0.0735 x 0.0025 = 0.0001837 mol

5. Divide g by moles of unknown to get molar mass unknown = g /mol = 0.050 g / 0.0001837 mol = 272.18 g/mol

What are the x and y coordinates of the cooling graphs?

x = time(seconds), y = temperature(celsius)

How to find freezing point depression in cooling graphs:

where the graph starts to flatten

Materials (7)

1. 1 large test tube

2. Hotplate

3. Beaker

4. Ring Stand

5. Utility Clamp

6. Rubber Stopper

7. Digital thermometer through the rubber stopper

What is the i value for Nacl?

i = 2 because NaCl (aq) = Na+ (aq)+ Cl- (aq) (2 particles)

How do you calculate molality from ΔTf? (Assume i = 1)

m = ΔTf / Kf

Solvent used in the experiment

Lauric Acid

Kf of lauric acid

3.90 °C/m

Why do we make cooling curves?

To determine the freezing point from the plateau region.

What does the flat part of the cooling curve represent?

The freezing point of the substance.

General steps to solve the calculations

1. Find ΔTf

2. Solve for molality with freezing point depression equation ΔTf​=iKf​m

3. Find Moles of Solute: mol solute=(molality) ×(kg solvent)

4. Calculate Molar Mass: g of unknown/ mol of solute

why do nonelectrolytes have i = 1

nonelectrolytes do not dissociate into ions when dissolved in a solvent so it produces 1 particle in the solution