Illustrated Guide to Home Chemistry Experiments

Chapter 8 Laboratory: Colligative Properties of Solutions 149

LABORATORY 8 .1:

dETERmINE moLAR mASS By BoILING poINT ELEvATIoN

Dissolving a nonvolatile solute in a solvent

increases the boiling point of that solvent by

an amount proportional to the quantity of the

solute. Although this phenomenon occurs with

any solvent and solute, for reasons of safety

and economy we’ll use water as the solvent for

this session, and sodium chloride (table salt)

and sucrose (table sugar) as the solutes.

SBSTITUTIU oNS ANd modIfICATIoNS

• You may use a kitchen stove burner for the heat
  source. Use a large tin can lid or similar item between
  the burner and glassware rather than putting the
  glassware in direct contact with the burner.


• You may substitute ordinary table salt for the
  sodium chloride.


• You may substitute ordinary table sugar for
  the sucrose.

RIREEqU d EqUIpmENT ANd SUppLIES

£ goggles, gloves, and protective clothing

£ balance and weighing paper

£ foam cups (6) and marking pen

£ beaker, 250 mL (1)

£ graduated cylinder, 100 mL

£ alcohol lamp or other heat source

£ tripod stand and wire gauze

£ beaker tongs

£ stirring rod

£ thermometer

£ boiling chips

£ sodium chloride (70.13 g)

£ sucrose (410.76 g)

£ tap water

dAR. Rm y CHERvENAk CommENTS:

This is also the reason for special high-altitude baking

instructions. Water boils at a lower temperature, so cakes

lose moisture more quickly, causing them to dry out and

collapse. The Canadian Rockies were high enough to

cause some baking disasters for me!

This lab has three parts. In Part I, we’ll determine the boiling point
of water under ambient pressure. In Part II, we’ll prepare solutions
of sodium chloride and sucrose of various molalities. In Part III,
we’ll determine the boiling points of those solutions.

CUTIOA nS

Although the chemicals used in this experiment are not

hazardous, you will be using open flame, so use caution

and have a fire extinguisher handy. Handle the hot liquids

used in this experiment with extreme care. If you use a

microwave oven to prewarm the solutions so that they’ll

come to a boil faster over the alcohol lamp, be careful not

to superheat the solutions. A superheated solution may

unpredictably boil violently, ejecting hot liquid from the

container. Always use a boiling chip to avoid superheating.

Wear splash goggles, gloves, and protective clothing.

z

POCEDURER

The boiling point of water under standard conditions is 100°C,
but, as is true of any liquid, the actual boiling point depends on
the ambient barometric pressure, which varies with weather
and altitude. If the barometric pressure is below the standard
pressure of 1000 millibars (mbar), the boiling point of water is
less than 100°C; if the pressure is greater than 1000 mbar, the
boiling point of water is greater than 100°C. Because barometric
pressure is not constant, the actual boiling point of water may
differ significantly from place to place and from day to day at a
particular place.

PRTI: A dETERNEmI THE BoILING poINT of wATER
UNdER AmBIENT pRESSURE