260 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. - You may use a commercial gas-generation bottle, gas-
collection trough, and gas-collection bottle instead of
the apparatus shown in Figure 14-5.
The Ideal Gas Law states that PV = nRT, where
P is absolute pressure, V is volume, n is the
number of moles, R is the ideal gas constant,
and T is the temperature in kelvins. If the values
for any four of these are known, the fifth can be
determined by solving algebraically. The value
of R, the ideal gas constant, depends on the
units used for volume and pressure. In SI units,
the value of R is 8.314472 joules per mole per
kelvin (J · mol–1 · K–1), which can also be stated
as 8.314472 L · kPa · mol–1 · K–1.
RIREEqU d EqUIpmENT ANd SUppLIES£ goggles, gloves, and protective clothing£ balance£ paper towel (quarter sheet)£ barometer (optional)£ thermometer£ graduated cylinder, 100 mL£ funnel£ Erlenmeyer flask, 250 mL£ stopper, 1-hole (to fit flask)£ glass tube, 75 mm (to fit stopper)£ plastic/rubber tubing (about 500 mm; to fit
glass tube)£ soft drink bottle, 2-liter, with cap£ large plastic storage tub (Rubbermaid “S” 17-liter
or similar)£ vinegar (75.0 g)£ sodium hydrogen carbonate (7.5 g)In this lab, we’ll use the ideal gas constant to determine the actual
percentage of acetic acid in distilled white vinegar. Most distilled
white vinegars are nominally 5% acetic acid by weight, but
industry standards permit that percentage to vary from 4.5% to
5.5%. To determine the actual mass percentage of acetic acid in
our sample, we’ll react a known mass of vinegar with an excess of
sodium hydrogen carbonate (sodium bicarbonate or baking soda)
to produce water, sodium acetate, and carbon dioxide gas by the
following balanced equation:
CH 3 CooH(aq) + NaHCo3(aq)
→ H 2 o(l) + CH 3 CooNa(aq) + Co 2 (g)
Stoichiometrically, one mole of acetic acid reacts with one
mole of sodium hydrogen carbonate to produce one mole each
of water, sodium acetate, and carbon dioxide. Because the
value for the ideal gas constant is known and the pressure and
temperature can be determined experimentally, if we capture
the carbon dioxide gas evolved by this reaction and measure its
volume, we can apply the Ideal Gas Law to determine the number
of moles of carbon dioxide produced and therefore the number
of moles of acetic acid that were present in the vinegar sample.
With that value, the mass percentage of acetic acid in the original
sample can be derived by a simple calculation.
Although you can purchase a gas-generating bottle, gas-
collection trough, and gas-collection bottle from a laboratory
supplies vendor, there’s no need to do so. I built my own gas-
generating bottle with a 250 mL Erlenmeyer flask, a one-hole
rubber stopper, and a short length of glass tubing. I used one of
our large plastic storage tubs that is ordinarily used for chemical
LABORATORY 1 4.4:
USE THE IdEAL GAS LAw To dETERmINE THE
pERCENTAGE of ACETIC ACId IN vINEGAR
storage for the gas-collection trough and an empty 2-liter
soft drink bottle for the gas-collection bottle. Figure 14-5 shows
my setup.