Vernier Software & Technology

# Using Freezing-Point Depression to Find Molecular Weight

## Introduction

When a solute is dissolved in a solvent, the freezing temperature is lowered in proportion to the number of moles of solute added. This property, known as freezing-point depression, is a colligative property; that is, it depends on the ratio of solute and solvent particles, not on the nature of the substance itself. The equation that shows this relationship is

$\Delta t = {K_f} \times m$

where Δt is the freezing point depression, Kf is the freezing point depression constant for a particular solvent (3.9°C•kg/mol for lauric acid in this experiment), and m is the molality of the solution (in mol solute/kg solvent).

## Objectives

In this experiment, you will

• Determine the freezing temperature of the pure solvent, lauric acid.
• Determine the freezing temperature of a mixture of lauric acid and benzoic acid.
• Calculate the freezing point depression of the mixture.
• Calculate the molecular weight of benzoic acid.

## Sensors and Equipment

This experiment features the following Vernier sensors and equipment.

### Option 2

You may also need an interface and software for data collection. What do I need for data collection?