Illustrated Guide to Home Chemistry Experiments

Chapter 8 Laboratory: Colligative Properties of Solutions 153

LABORATORY 8 .2:

dETERmINE moLAR mASS By fREEzING poINT dEpRESSIoN

Just as a dissolved solute increases the boiling

point of a solvent by an amount proportional

to the quantity of the solute, it also decreases

the freezing point of that solvent. The same

formula is used to calculate either value, except

that a negative sign is added for freezing point

depression to account for the fact that the

freezing point is lowered rather than raised.

RIREEqU d EqUIpmENT ANd SUppLIES

£ goggles, gloves, and protective clothing

£ beaker, 600 mL (1)

£ test tubes (6)

£ thermometer

£ stirring rod

£ chilled solutions A through f from preceding lab

£ chipped ice sufficient to half fill beaker

£ sodium chloride sufficient to make a 15 mm to

25 mm layer on top of crushed ice

SBSTITUTIU oNS ANd modIfICATIoNS

• You may substitute any glass or plastic wide-mouth
  container with a capacity of 1 pint (~500 mL) to
  1 quart (~1 L) for the 400 mL or 600 mL beaker.


• You may substitute table salt or rock salt for the
  sodium chloride.

CUTIOA nS

Wear splash goggles, gloves, and protective clothing.

z

Boiling point elevation = ΔTb = ikbm

freezing point depression = ΔTf = –(ikfm)

In fact, some sources specify freezing point depression constants,
Kf or Kfp, as negative signed values, eliminating the need to reverse
the sign in the formula. In either case, the calculations are done the
same way for boiling point elevation and freezing point depression.
Only the values of the constants differ. For example, the Kb for
water is 0.512°C/molal and the Kf for water is 1.858°C/molal.
In other words, assuming a van’t Hoff factor of 1 for the solute, a
1 molal solution has a boiling point 0.512°C higher than pure water,
and a freezing point 1.858°C lower. A 2 molal solution has a boiling
point 1.024°C higher than pure water, and a freezing point 3.716°C
lower. And so on.

In this laboratory, we’ll use the sodium chloride and sucrose
solutions prepared in the preceding lab to test the effect of
dissolved solutes on freezing points.

POCEDURER
This lab has two parts. In Part I, we’ll prepare an ice/salt bath
and determine the freezing point of water. In Part II, we’ll
determine the freezing points of the solutions we prepared in
the preceding laboratory.

PRTI:A RE REp pA THE ICE BATH ANd dETERmINE
THE fREEzING poINT of wATER
Unlike the boiling point of water, which varies significantly
with ambient local pressure, the freezing point of water is
almost unaffected by pressure. But thermometers vary in
accuracy, so before we test freezing point depressions in the
sodium chloride and sucrose solutions, we’ll calibrate our
thermometer by using it to determine the freezing point of
water. (For example, if our thermometer indicates a freezing
point for pure water of +0.5°C and we subsequently measure
a depressed freezing point at –2.5°C, the actual freezing point
depression is 3.0°C rather than 2.5°C.)

Because the presence of a solute reduces the freezing point

of an aqueous solution below 0°C, the freezing point of pure

water, we need a way to cool the solutions below 0°C. In an

elegant application of using freezing point depression to test

freezing point depression, we’ll use the phenomenon itself

to provide the conditions necessary for the test. We’ll make

an ice/salt bath by mixing crushed ice with sodium chloride.

The phenomenon of freezing point depression means that

the temperature of this ice/salt bath will be lower than the

temperature of a pure ice bath.