1. Determination of molar mass
1.a. Cryoscopic determination of molar mass of naphthalene
TASK: Determine the molar mass of naphthalene. Measure the freezing-point
temperature of naphthalene solution in benzene solvent for three different
solute concentration.
LABORATORY AIDS AND CHEMICALS: Apparatus for cryoscopy (see xxxxx Fig. 2),
digital thermometer, burette for measuring volumes of volatile liquids (25 cm3
),
analytical balance, weighting bottle (25 cm3
), spoon, benzene, naphthalene, ice,
stopwatch.
INSTRUCTIONS: Determine the benzene cooling curve (XXXXFIG.3) as instructed
in the introductory chapter. Apply 20 cm3
of pure benzene. Weight benzene
liquid in weighting bottle (need for 4 significant digits). Repeat the curve
measurement twice.
Weigh about 0.2 g (need for 4 significant digits) of naphthalene into small weighting
bottle and put it in a cryoscopic tube. Dissolve the naphthalene and determine the
freezing-point of the solution twice.
In the same way, measure the freezing-point temperature of other solutions at solute
concentrations corresponding to the total addition of 0.4 and 0.6 g of naphthalene
(i.e., to the previous amount of naphthalene add 0.2 g). It all in original 20 cm3
of
benzene.
REPORT: Systematic deviation of the digital thermometer. Table 1: for each
experiment: benzene weight, naphthalene weight, experimental
benzene/solution freezing point temperature, mean benzene freezing-point
temperature and the calculated molar mass of naphthalene. Common graph 1:
cooling curves of pure benzene and naphthalene solutions.
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