MHS AP Chemistry
Colligative Properties and Molecular Weight

The purpose of this activity is to determine the molecular weight of an unknown substance by measuring the freezing point depression produced in a known solvent. The molecular weight will be used to identify the unknown substance.

1. The solvent is naphthalene. The freezing point depression constant for this substance is k = -6.94 C°/m. The solute will be announced in class.
2. Measure the mass and melting point of naphthalene.
3. Add some of the unknown solute (no more than about 1/20th of the solvent mass). Measure the new melting point, and determine the mass of unknown added.
4. Use the formula DT = kfm [eq. 13.12] and the definition of molality to calculate the molecular weight of the unknown solute.
5. Use the measured molecular weight of your unknown to identify it from the following list: acetone, anthracene, benzophenone, butane, chlorobenzene, naphthalene, sodium chloride, toluene, xylene.
6. Look up the actual melting point of pure naphthalene.

Write Up

• Each person must pass in their own lab report.
• Each report should have a complete Title Section & Data Table.
• Show every calculation completely & clearly (names, values, and labels).
• Determine the % error in the measured melting point of pure naphthalene.
• Determine the % error in the measured molecular weight of your unknown.
• Remember that only data goes in the data table.

This lab is due on ______________. After that there will be a penalty of 2 points per day late.

[Syllabus 13][MHS AP Chem page]