Freezing Point Depression & Molecular Weight
The purpose of this activity is to determine the molecular weight of a known
substance by measuring the freezing point depression produced in a known solvent.
The calculated molecular weight will be compared to the "official"
- The solvent is p-dichlorobenzene (also known as 1,4-dichlorobenzene). The
freezing point depression constant for this substance is k = 7.10 C°/m.
The solute can be either naphthalene (C10H8)
or iodine (I2). You get to choose.
- Calculate the molecular weight of your chosen solute.
- Measure the mass and melting point of p-dichlorobenzene in a medium test
- Add some of the solute (no more than about 1/20th of the solvent mass).
Measure the new melting point, and determine the mass of solute added.
- Use the formula DT = km and the definition
of molality to calculate the moles, then molecular weight of the solute.
- If your answer in #2 is considered "correct," what is your percent
error in #5?
- Each person must pass in their own lab report.
- Each report should have a complete and beautiful Title Section & Data
- Show every calculation completely & clearly (names, values, and labels).
- Include the score sheet when you pass in your lab.
- Remember that only data goes in the data table.
Point Depression & Molecular Weight score sheet][MHS