Chapter 21 Laboratory: Synthesis of Useful Compounds 377PRTIII: A dETERNEmI dENSITy ANd fREEzING
poINT of THE pRodUCT
Our product is a few mL of semi-pure methyl salicylate. If we
had the equipment needed for microscale distillations, we could
further purify our raw product by distillation. We don’t have
that equipment, so we’ll test our product as-is to determine its
density and freezing point.
We know that the density of pure methyl salicylate is 1.1825
g/mL, so the density of our product will give us some idea of
its purity. That’s not sufficient, however. Even if the density of
our product is very close to 1.1825 g/mL, for all we know our
product might be a small amount of methyl salicylate mixed
with a large amount of some impurity that has the same
density. The freezing point of our product will give us a better
idea of its purity. Pure methyl salicylate freezes (or melts) at
–8.3°C. Furthermore, the sharpness of the freezing/melting
point gives a good indication of a product’s purity. Pure
substances tend to freeze/melt very sharply, at one particular
temperature. Impure substances freeze/melt more gradually,
over a range of a few degrees.
- If you have not already done so, put on your splash
goggles, gloves, and protective clothing. - Weigh a clean, dry, empty 50 mL beaker and record its
mass to 0.01 g on line E of Table 21-1. - Draw up as much as possible of your product into the
10.0 mL Mohr or serological pipette. Record the volume
of product (if possible, to 0.01 mL) on line F of Table 21-1.
(If you somehow produced more than 10 mL of product,
use as close as possible to a 10.00 mL sample.)
4. Transfer the measured product to the 50 mL beaker.
Reweigh the beaker and record the combined mass of
the beaker and methyl salicylate to 0.01 g on line G of
Table 21-1. Subtract the mass of the empty beaker from
the combined mass of the beaker and sample, and enter
that value to 0.01 g on line H of Table 21-1. Divide the
mass of the sample (line H) by the volume of the sample
(line F) to determine the density of the sample. Enter
that value on line I of Table 21-1.
5. Transfer as much as possible of the sample to a test
tube. Immerse the test tube in the ice bath, and use
a thermometer (carefully) to stir the contents of the
test tube. When the temperature of the sample falls to
0°C, check the contents of the tube frequently to see
whether solid crystals of methyl salicylate have begun to
form. (You’ll need to remove the tube from the ice bath
and examine it against a strong light to detect the first
formation of crystals.) Record the temperature at which
crystals just begin to form on line J of Table 21-1.
6. Continue cooling the contents of the tube until the
methyl salicylate freezes solid. (Leave the thermometer
embedded in the sample.) Record that temperature on
line K of Table 21-1.
7. Remove the test tube from the ice bath and allow it to
begin warming. Record the temperature at which the
frozen sample first begins to liquefy on line L of Table 21-1.
8. Allow the tube to continue warming. Stir gently with the
thermometer, and record the temperature at which the
sample again becomes completely liquid on line M of
Table 21-1.
dISpoSAL:
The methyl salicylate produced in this lab is not safe
for human consumption under any circumstances. It
contains numerous impurities, some of which may be
toxic. (or perhaps I should say even more toxic than
methyl salicylate itself.) do not taste it or allow it to
contact your skin. discard it by flushing it and all other
waste solutions from this lab down the drain with plenty
of water. Alternatively, you can use your crude methyl
salicylate in a scent candle by mixing it with paraffin.FIGURE 21-2: Determining the freezing point of the semi-pure
methyl salicylate