Illustrated Guide to Home Chemistry Experiments

Chapter 8 Laboratory: Colligative Properties of Solutions 151

dISoALp S : Retain all of the solutions for use in the

following laboratory session. Allow all of the solutions

to cool to room temperature, and then place them in the

refrigerator to chill them. A typical refrigerator maintains

a temperature of about 5 °C. Having the solutions at this

temperature reduces the time required for the freezing

point determinations in the following laboratory session.

(I violate my general rule of avoiding mixing laboratory

materials with kitchen materials because these solutions

contain only food compounds.)

FIGURE 8-1: Determining the boiling point of a solution

oALpTIoN ACTIvITIES

If you have time and the required materials, consider

performing these optional activities:

• Repeat the experiment, using various
  concentrations of engine coolant (antifreeze). Most
  commercial antifreezes contain ethylene glycol (FW
  62.07 g/mol), propylene glycol (FW 76.09 g/mol),
  or a mixture of the two. Assume that the solute is
  molecular rather than ionic, and use the boiling
  point elevation values you observe to calculate the
  formula weight of the commercial coolant solution.
  (Note that ethylene glycol is extremely toxic and
  must be handled with respect and disposed of
  properly. Propylene glycol is less toxic, but still must
  be handled properly.)


• Repeat the experiment, but dissolve sufficient
  sodium chloride and sucrose in the initial 100 g of
  water to make the solution 1.5 molal with respect
  to both solutes. (Warm the water if necessary to
  dissolve all of the solids.) Calculate the expected
  boiling point elevation and then determine the
  actual boiling point elevation for the solution.


• Repeat the experiment, using an unknown solute
  provided by a parent, teacher, or lab partner. (In
  order to maximize the boiling point elevation, the
  unknown solute should be of as low a formula weight
  as possible, and as high a solubility as possible.
  Solutions of low molality are difficult to test with a
  standard thermometer, because the accordingly
  small value for boiling point elevation is subsumed
  by measurement error. ) Record the value for boiling
  point elevation, and calculate the formula weight of
  the solute assuming a van’t Hoff factor of 1.

TABLE 8-1: Boiling point elevation—observed and calculated data

Beaker/solutionBoiling pointCalculated formula

weight

Water ______.__°C

A. Sodium chloride,

6 mol/kg ______.__°C_________.___ g/mol

B. Sodium chloride,

3 mol/kg ______.__°C_________.___ g/mol

C. Sodium chloride,

1.5 mol/kg ______.__°C_________.___ g/mol

D. Sucrose,

6 mol/kg ______.__°C_________.___ g/mol

E. Sucrose,

3 mol/kg ______.__°C_________.___ g/mol

F. Sucrose,

1.5 mol/kg ______.__°C_________.___ g/mol