Page 38

Partners: Jeff Somvorachith

11. Molecular Mass Determination Using Boiling and Freezing Point

Purpose: 

To practice calculating the molecular mass of solutes by comparing boiling and freezing point of two solutes with known concentrations.

Procedure:

A. 

1. 1. 1. Measure 50g of distilled water into a 125ml flask. set up the flask on a hot plate and hang a thermometer in the flask using a clamp, not letting the thermometer touch the flask. Record the temperature to the nearest 0.5 ^oC after heated.
      2. Prepare a solution of 10g potassium nitrate in 50g of distilled water. Determine the boiling point of the solution using the equation m=ΔT_BP/K_BP where K_BP=0.512 ^oC, m=molality, and ΔT=change in temperature. Dispose of the solution by rinsing it down a sink.
      3. Repeat step 2, but replace the KNO₃ with C6H12O₆, Dextrose.

B. 

1. 1. 1. Dissolve 5g of KNO₃ in 50g of water in a 125ml flask. Place the flask in a beaker with rock salt, with the beaker wrapped in a paper towel. Wait until crystals form in the solution and becomes slushy. Record the temperature. Rinse the solution down a drain.
      2. Dissolve 10g C₆H₁₂O₆ in 50g of distilled water. Place the flask with the solution in a beaker filled with ice an rock salt and wait until the solution crystalizes. Measure the temperature and rinse down a drain.