Freezing Lab 

Overview of Lab Activity

• The lab focuses on determining the molar mass of a solute based on temperature changes (colligative properties).

• The solute used is lauric acid, a fatty acid commonly found in lipids and fats.

Lauric Acid and Glycerol

• Lauric acid is a long-chain fatty acid, typically nonpolar, which does not mix well with water.

• Glycerol (or glycerin) is a three-carbon alcohol with hydroxyl (OH) groups attached, forming the backbone of triglycerides in lipids:

  • Each carbon of glycerol bears one hydroxyl group.

• Fatty acids link to glycerol through dehydration synthesis, forming triglycerides, which are the main storage form of fats.

Properties and Behavior of Lauric Acid

• Lauric acid has a melting point around 40°C.

• Its waxy nature means it behaves like wax and can be challenging to clean if spilled, often requiring physical scraping rather than washing with water due to its nonpolar characteristics.

Experimental Procedure

• Students will work in teams, with each team measuring temperature changes in solid and dissolved lauric acid:

  • One group uses pure lauric acid while another mixes lauric acid with benzoic acid.

  • Benzoic acid will dissolve in the lauric acid but does not melt at the temperatures involved in the experiment (melting point above 100°C).

Measuring Freezing Point

• The goal is to identify the freezing point of both samples:

  • Temperature is measured using probes, and careful setup is necessary to avoid direct contact with the test tube's bottom to accurately gauge the liquid's temperature.

  • Students will record the temperature changes until they observe the plateau indicative of the freezing point, where temperature remains constant as the solidifies.

Colligative Properties and Calculations

• The lab utilizes molality (m), defined as moles of solute per kilogram of solvent:

  • The provided mass of lauric acid (12.4g) and benzoic acid (1.6g) will facilitate these calculations.

• Students are expected to use temperature change (delta t) to determine molality and subsequently calculate the molar mass of the benzoic acid.

Group Dynamics and Data Collection

• Teams will collect and share temperature data concurrently, analyzing results using Vernier graphical analysis (VGA) software.

  • The data will be saved and manipulated in Google Sheets for better visualization and assessment.

Important Points for Students

• Critical to ensure that the temperature probes are positioned correctly to avoid measuring the test tube’s temperature rather than the sample’s.

• The melting/freezing point graphs will show noteworthy leveling off, indicating phase changes where temperature does not change despite ongoing energy transfer.

• The lab process allows one to engage with textbook problems and reinforces the importance of careful procedure adherence for accurate outcomes.

Summary of Steps for Completion

• Prepare setups with boiling water in beakers to melt samples.

• Ensure temperature probes do not touch the test tube's bottom.

• Monitor temperature continuously and record data carefully.

• Collaborate with peers for data collection and analysis, focusing on both physical and graphical representations of temperature changes.