98 DIY Science: Illustrated Guide to Home Chemistry Experiments
sure that the indicated temperature does not climb
above the stable value; if it does, that indicates that the
ethanol has been eliminated from the distillation vessel
and what you’re transferring to the receiving vessel is
pure water. If that occurs, start again using more of the
original ethanol mixture.)- Allow the distillate to cool to room temperature.
Transfer 100.0 mL of the distillate to the volumetric flask,
weigh the flask and contents, and record the mass on line
E of Table 6-2.
Subtract line A from line E to determine the mass of the
distillate, and record this value on line F of Table 6-2.
Divide line F by 100 to give the density of the distillate
in g/mL and record this on line G.
FIGURE 6-2: My distillation apparatusIn this laboratory, we’ll use distillation to increase the concen-
tration of an ethanol solution. At 25°C, pure water has a density
of 0.99704 g/mL and pure ethanol a density of 0.78736 g/mL.
Solutions of ethanol and water have densities between these
figures. If you add the densities of the pure liquids and divide
by two, you get 0.89220 g/mL, which you might assume is the
density of a 50/50 ethanol-water mixture. As it turns out, that’s
not true. Ethanol and water do not mix volumetrically; that is,
if you mix 100 mL of pure ethanol with 100 mL of pure water,
you do not get 200 mL of solution, for the same reason that
dissolving 100 mL of sucrose in 100 mL of water does not yield
200 mL of solution.
Nonetheless, it’s possible to determine the concentration of
ethanol by measuring the density of the solution and comparing
that value to ethanol-water density tables available in the CRC
handbook and similar publications. We’ll measure the density
of the starting solution and the resulting distillate and compare
POCEDURER
1.f you have not already done so, put on your splash I
goggles, gloves, and protective clothing.
- Weigh the volumetric flask on your balance and record
the mass in on line A of Table 6-2. - Transfer 100.00 mL of ethanol to the volumetric flask and
reweigh the flask. Record the mass on line B of Table 6-2. - Subtract line A from line B and record the result (the
mass of the 100 mL of ethanol) on line C. Divide line C by
100 to give the density of the ethanol in g/mL and record
this value on line D. - Empty the ethanol from the volumetric flask into the
distillation vessel and add about 100 mL more ethanol.
Add one or two boiling chips to the flask. - Reassemble the distillation apparatus, using the 250 mL
beaker as the receiving vessel. - Turn on the hotplate and bring the ethanol to a gentle
boil. As the ethanol begins to boil, the temperature
indicated by the thermometer in the distillation
apparatus will begin to climb rapidly until it stabilizes at
the temperature of the ethanol-water vapor. - Continue the distillation until more than 100 mL of
distillate has collected in the receiving vessel. (Make
DISPOSAL: you can retain the distilled ethanol and use
it as fuel for your alcohol lamp or for any other purpose
that requires denatured ethanol where concentration is
not critical, or you can safely pour it down the drain.
If you retain the distillate, label it properly.SBSTITUTIU oNS ANd modIfICATIoNS (continued)- Most drugstore ethanol is 70% by volume (about
64.7% by weight), but 90% and 95% concentrations
are also common. If you start with more concentrated
ethanol, dilute it with water to 70% before you begin
the distillation.
those values to published values to determine the ethanol
concentrations of the two solutions.Figure 6-2 shows the distillation apparatus that I used for this
laboratory session. Note that both flasks are securely clamped to
prevent tipping, and that the stopper in the receiving flask (right)
has an open hole to vent the flask and prevent pressure buildup.
The receiving flask sits in a large container of ice water. As the
alcohol vapor passes through the tubing and into the receiving
flask, it condenses immediately when it contacts the cold air in
the receiving flask.