MHS AP Chemistry
Colligative Properties and Molecular Weight #2

UPDATE THIS LAB!

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

1. The solvent is naphthalene (also known as 1,4-dichlorobenzene). 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]