20210406173853online Chem 1412 Mm Determination With Excel Data Tables 7 3 20

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Online CHEM 1412 Molar Mass Determination

Dallas College p 1

Objectives:

1. To introduce colligative properties of solutions

2. To determine the freezing point liquids

3. To use experimental freezing point depression data to calculate the molar mass of

an unknown nonelectrolyte solute

Materials:

● Large test tube*

● Thermometer

● Stir rod

● 250 mL beaker

● 100 mL graduated cylinder

● Unknown F 2g

● Distilled water

● Ice

● NaCl (s) 100 g (provided by student)

*the large test tube in this kit is actually a centrifuge tube and looks like this:

Introduction:

Colligative properties of a solution are properties that depend upon the concentration of

solute particles (ions, atoms, or molecules), but not upon the identity of the solute.

There are four colligative properties (see lecture notes for details):

1) freezing point depression

2) boiling point elevation

3) vapor pressure lowering

4) osmosis

This lab focuses on the freezing point depression of an aqueous solution. When a solute

is added to a pure solvent, the freezing point of the resulting solution is lower than that of

Online CHEM 1412 Molar Mass Determination

Dallas College p 2

the pure solvent. This observation is known to chemists as freezing point depression. An

application of this effect is the use of salt to melt ice on the highways in the winter

months.

At the molecular level, the freezing point depression can be explained by using the

Kinetic Molecular Theory (KMT, see lecture notes). When the temperature of a solution

is lowered, the average kinetic energy of the molecules decreases. Consequently, as the

temperature is lowered, the molecules slow down. When the intermolecular attractive

forces between the molecules overpower the forces of repulsion caused by the kinetic

energy, the molecules get closer to each other and form a solid.

In a solution, the solute particles interfere with the solidification process described above

by interacting with the solvent molecules. As a result, a lower temperature is required to

cause the solvent particles to form a solid.

The relationship between the changes in the temperature and the concentration of the

solution is given by the following formula:

△T

= i K

where

● m is the molality of the solution (see lecture notes for more details)

● i is the van't Hoff factor (i = 1 for a nonelectrolyte)

● K

is the molar-freezing-point-depression constant (K

= 1.86°C/m for water).

△T

can be found experimentally by determining the difference between the freezing

points of the pure solvent and the solution.

△T

must be determined by extrapolation of graphic data because of the phenomenon of

supercooling, which is the sudden lowering of freezing point of a solution beyond its

actual freezing point.

Procedure:

PART I: Freezing Point of the Solvent (water)

1. Prepare an ice bath as follows:

● Fill a 250 mL beaker with crushed ice to the 200 mL line.

● Add 100 g of table salt to the ice.

● Add tap water until 2/3 of the ice is covered.

● Stir vigorously with your stirring rod.

2. Measure 20.0 mL distilled water into a large test tube.

3. Insert a thermometer into the liquid in the large test tube.

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Online CHEM 1412 Molar Mass Determination Objectives: 1. To introduce colligative properties of solutions 2. To determine the freezing point liquids 3. To use experimental freezing point depression data to calculate the molar mass of an unknown nonelectrolyte solute Materials: ● Large test tube* ● Thermometer ● Stir rod ● 250 mL beaker ● 100 mL graduated cylinder ● Unknown F 2g ● Distilled water ● Ice ● NaCl (s) 100 g (provided by student) *the large test tube in this kit is actually a centrifuge tube and looks like this: Introduction: Colligative properties of a solution are properties that depend upon the concentration of solute particles (ions, atoms, or molecules), but not upon the identity of the solute. There are four colligative properties (see lecture notes for details): 1) 2) 3) 4) freezing point depression boiling point elevation vapor pressure lowering osmosis This lab focuses on the freezing point depression of an aqueous solution. When a solute is added to a pure solvent, the freezing point of the resulting solution is lower than that of Dallas College p1 Online CHEM 1412 Molar Mass Determination the pure solvent. This observation is known to chemists as freezing point depression. An application of this effect is the use of salt to melt ice on the highways in the winter months. At the molecular level, the freezing point depression can be explained by using the Kinetic Molecular Theory (KMT, see lecture notes). When the temperature of a so ...
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Tags: Freezing Point moles of solute molality of solution unknown nonelectrolyte temperature of liquid