Solute Effect on Temperature

Purpose:

    The effect of various salts on boiling point (Bp) and freezing point depression is seen in this experiment. You will try to relate the number of ions produced to how much the boiling point is raised and the freezing point is depressed. Percent error can be determined by using chemical reference tables.

Materials:

  • graduated cylinder

  • 250 ml beaker

  • thermometer

  • hot plate or bunsen burner with stand, ring and screen.

  • ice chips

  • various salts (no ammonium compounds)

Procedure:

Put on goggles and aprons after reading the MSDS sheets (click on internet icon to get MSDS sheets).

    Boiling Point Elevation

  1. Boil 110 ml of distilled water and record the temperature. Some of the water will evaporate before adding the chemical.

  2. Add enough chemical to make a 2 molal solution assuming 100 ml of solution.

  3. Stir the chemical until it dissolves. When it is boiling record the temperature again.

  4. Measure the actual amount of solution.

  5. Make a temperature correction based upon the actual amount of solution.

    Freezing Point Depression

  1. Mix up 50 ml of a 1 molal solution of chemical. Place this in a 250 ml beaker.

  2. Put the beaker into an ice bath. An ice bath is made by filling a container larger than the beaker with ice and then pouring salt on the ice. As the ice melts, more salt and ice should be added until the experiment is over.

  3. Record the temperature when the solution in the beaker starts to freeze after slow mixing.

  4. Compare your results to the expected value.

Discussion:

Molal freezing point depression is 1.86oC and the molal boiling point elevation is 0.52oC.

For example the effect of calcium chloride CaCl2 would produce three ions: one calcium (Ca+2) and two chlorides (Cl-1). Therefore, a one molal solution should lower the freezing point 5.58oC (3 ions times 1.86).

List the chemicals in order from the one that changed the temperature the least to the one that had the greatest effect. Next to the chemicals put the number of degrees difference compared to the original boiling point or freezing point of water. Next to the degrees difference put the accepted values. Give the percent error based only on the temperature difference and then discuss the results.

Explain your answer in relation to sodium chloride or calcium chloride. Which chemical is better to use by the highway department in order to melt snow on the roads.

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