264 DIY Science: Illustrated Guide to Home Chemistry Experiments
SBSTITUTIU oNS ANd modIfICATIoNS- If you do not have a barometer, you may use the current barometric pressure
broadcast by a local TV or radio station, but see the note on atmospheric
pressure versus barometric pressure in Laboratory 14.1. - Use the largest Erlenmeyer flask you have that weighs less than the
maximum capacity of your balance. If you use a larger flask, increase the
amount of acetone proportionally. For example, with a 500 mL flask, use
10 mL of acetone. - This experiment requires slowly increasing the temperature of a small
amount of acetone contained in a large Erlenmeyer flask until the acetone
boils. The easiest and most consistent way to do this is to use an old kitchen
pot or similar container as a warm water bath. At standard pressure,
acetone boils at about 56.5°C. Suspend the flask inside the water bath
container, and add sufficient water at about 50°C to immerse as much as
possible of the flask under the surface of the water. Allow a few minutes for
the flask and its contents to stabilize at the temperature of the water bath,
and then begin adding small amounts of boiling water with stirring until the
acetone in the flask begins to boil.
According to the Ideal Gas Law, PV = nRT.
Rearranging this equation to put n, the number
of moles, on one side gives us:
n = (pv)/(RT)
RIREEqU d EqUIpmENT ANd SUppLIES£ goggles, gloves, and protective clothing£ balance£ barometer (optional)£ thermometer£ graduated cylinder, 100 mL£ Erlenmeyer flask, 250 mL or larger (see
Substitutions and modifications)£ beaker, 250 mL£ ring stand£ support clamp (for flask)£ warm water bath (see Substitutions and
modifications)£ pin or needle£ aluminum foil£ acetone (~ 5 mL)For a sample of a gas in a container, R (the ideal gas constant) is
known, and the pressure (P), volume (V), and temperature (T) are
easy to determine experimentally. With these four values known,
determining n, the number of moles of gas in the container, is a
simple calculation. Because the number of moles in the sample
(n) equals the mass of the sample divided by the molar mass of
the substance, the only additional datum we need to calculate the
molar mass of the substance is the mass of the sample.
In 1826, the French chemist Jean Baptiste André Dumas
developed a method and an apparatus for determining molar
mass from vapor density. The original apparatus from his
illustration in his 1826 report is shown in Figure 14-6.
LABORATORY 1 4.5:
dETERmINE moLAR mASS fRom vApoR dENSITy
FIGURE 14-6:
Apparatus used by Dumas in 1826 for
determining molar mass from vapor density