Experiment Objective: Understand how a solute affects the freezing and boiling points of a solvent.
Pure substances have specific melting, freezing, and boiling points.
Adding a solute to a solvent decreases its freezing point and increases its boiling point.
Example: Salt on roads lowers the freezing point, preventing ice formation at 32°F.
Key Concepts
Solution: Consists of a solute (dissolved substance) and a solvent (substance in which solute is dissolved).
Freezing Point Depression Equation: ΔTf = -m * kf
ΔTf: Change in freezing point.
m: Molality of the solution.
kf: Freezing point depression constant, specific to the solvent.*
Definitions
Molality (m): Moles of solute per kilogram of solvent.
Solute: Substance being dissolved, also the unknown in the experiment.
Solvent: Lauric acid in this experiment.
Experiment Steps
Measure Freezing Point of Pure Solvent (Lauric Acid)
Weigh lauric acid in a test tube.
Immerse in boiling water to melt.
Record temperature until it solidifies, noting the longest constant temperature as the freezing point.
Add Solute and Measure Freezing Point
Weigh the unknown solute.
Add to melted lauric acid.
Repeat temperature recording process to find the freezing point of the solution.
Calculations
Determine Molality:
From ΔTf, solve for molality.
Use the molality to find moles of the solute.
Find Molar Mass:
Molar Mass = Grams of solute / Moles of solute.
Grams are measured directly; moles are calculated from molality and known mass of the solvent.
Important Reminders
The solute is the unknown; the solvent is lauric acid.
Record temperature accurately, focusing on the constant temperature phase during solidification.
Handling Equipment
Taring the Balance:
Zero the balance with beaker and test tube before weighing.
Using a Rubber Stopper:
Ensure proper fit to avoid breakage.
Thermometer Use:
Ensure full immersion for accurate temperature readings.
Safety Precautions:
Use tongs when handling hot materials.
Conclusion
The experiment demonstrates the principle of freezing point depression and allows for the calculation of molar mass of the unknown solute. Repeat measurements for accuracy. Follow safety procedures when working with hot equipment.