Molar Mass of a Molecular Compound by Freezing-Point Depression: A "Green" Chemistry Activity

By Mike Isley
Carolina Product Developer

Most freezing-point depression labs determine the molar mass of a solute by using organic compounds for solvents and solutes. The reason for using these compounds is that they have freezing points above room temperature, but the downside is that they also have odor and disposal issues. And, if students accidently pour them down the sink while they are still warm, they will solidify in the plumbing as they cool down to room temperature.

To address those concerns, this lab uses an aqueous solution of a green compound (sucrose, C₁₂H₂₂O₁₁, 342 g/mole) for determining its molar mass by the freezing-point depression of water. Sucrose is considered a green compound for this lab because it requires no hazardous material disposal and produces no toxic fumes. Unlike the compounds it replaces, it requires cooling (provided by a salt-ice bath) to reach the freezing point of its aqueous solution. This lab is designed for 30 students working in pairs. Download the Student Worksheet.

Teacher prep

• Have the following materials at each lab station:
  • 250-mL Beaker
  • Laboratory Thermometer
  • Graduated Cylinder
  • 20.0 g Salt (in a weigh boat or filter paper)
  • Crushed Ice (or small ice cubes)
  • Standard Test Tube
  • Stopwatch (or timer)
• Have a weighing station with one or more electronic balances readable to at least 0.1 g.
• Have a 250-mL beaker beside each balance filled with sucrose and labeled as “Compound X.”
• Have spatulas or spoons in each sucrose beaker and weigh boats or pieces of filter paper to mass unknown X.

Answers to sample calculation

1. Change in freezing temperature between pure water and the glucose solution, ΔT = 4.0° C
2. Molality (m) of the solute in the solution
   m = ΔT/K_f = 4.0° C/1.86° C•kg/mol = 2.2 mol/kg
3. Moles of solute in solvent
   Moles of solute = m x kg of solvent = 2.2 mol/kg x 0.010 kg = 0.022 mol
4. Experimental molar mass of solute
   Molar mass = grams of solute/moles of solute = 4.2 g/0.022 mol =191 = 190 g/mol.
5. The accepted molar mass of glucose (C₆H₁₂O₆) = 180 g/mol
6. Percentage of error: x 100 = 5.6%

Further explorations

Carolina™ Chemistry has everything you need to further your class’s explorations of molar mass by freezing-point depression. For example, our Molar Mass by Freezing-Point Depression AP® Chemistry Kit is perfect for AP® Chemistry topic 4. The Carolina™ AP® Chemistry CD-ROM Series also supports this topic with a virtual interactive laboratory. Our exclusive Inquiries in Science® Chemistry Series offers the Observing Colligative Properties Kit, which addresses freezing point and other topics.