Name
Lab Day
Analysis for Substances in the Atmosphere
Introduction:
Some of the more important gases which are found in the
atmosphere are oxygen (O2), hydrogen (H2), carbon dioxide (CO2),
methane (CH4), nitrogen (N2), nitrogen dioxide (NO2), water vapor (H2O),
and sulfur dioxide (SO2). In this experiment, you will obtain some
qualitative evidence of the presence of some of these gases, and a
quantitative measurement of the amount of oxygen in the atmosphere.
Quantitative measurement of a constituent in a gaseous mixture is
usually expressed as percent by volume, a figure which also expresses
the relative number of molecules of that constituent in the mixture.
Experimental:
Qualitative observations of water vapor. Place several granules of
anhydrous calcium chloride on a watch glass and leave it exposed to the
air. At the end of the lab period, observe any change that has occurred.
(In extremely dry weather, no water at all may be absorbed.)
Qualitative observations of carbon dioxide. Place about 15 mL of
0.1 M Ba(OH)2 in a large Erlenmeyer flask (500 mL). Stopper the flask
and shake it for a minute. Observe the change. (The reaction is the
formation of a precipitate of barium carbonate, BaCO3. The test is very
sensitive for CO2).
Qualitative observations of nitrogen. Place about 15 cm of
magnesium ribbon, packed compactly, into a crucible. Cover the crucible
with a crucible lid. Heat intensely with a Bunsen burner for a good 10
minutes. Cool the crucible, and replace the lid with a small watch glass
which has a moist piece of red litmus paper attached to the underside.
Lift the watch glass just enough to add a couple of drops of water to the
white residue. Warm the crucible slightly. Note any change in the litmus
color. The reaction primarily forms magnesium oxide. Along with this
product, some of the magnesium reacts with nitrogen to form
magnesium nitride, Mg3N2. This product forms ammonia gas, NH3 and
magnesium hydroxide, Mg(OH)2 by reaction with water.
Mg3N2(s) + 6 H2O → 3 Mg(OH)2(s) + 2 NH3(g)
NH3(g) + H2O → NH4OH(aq)
You may smell the odor of ammonia gas above the moist, white, solid
magnesium hydroxide.
2
Quantitative determination of oxygen in the air. Prepare the
apparatus shown below:
Equalize the levels
inside and outside
the flask and replace
the clamp.
Pyrogallol solution
6M NaOH
Remove the stopper and fill the flask and the test tube (still in the
flask) with tap water. Temporarily disconnect the rubber tube and clamp,
and firmly re-insert the stopper and glass tube into the flask. Make sure
that no air bubbles are trapped. Remove the stopper and glass tube and
measure the volume of water (include the water in the glass tube) with
your graduated cylinder to obtain the capacity of the apparatus. Record
this volume on your report sheet.
Empty the apparatus, but do not dry it. Measure 20 mL of 6M
NaOH into the flask, and 5–6 mL of a solution of 30% pyrogallol
(pyrogallic acid) into the test tube. The pyrogallol solution is made by
dissolving 2.0 grams of pyrogallic acid in 5 mL of water. Record the exact
volume of the pyrogallol used. Firmly close the flask with the stopper and
glass tube and re-attach the rubber tubing without the clamp. Tightly
clamp the rubber tube just above the glass tube. Turn the flask upside
down and allow the two solutions to mix. Swirl the contents every couple
of minutes for 15 minutes. After 15 minutes, the oxygen in the flask
should have been completely absorbed by the alkaline pyrogallol. Put
the flask in a large pneumatic trough, bottom side up and open the
clamp to allow water to flow into the flask to replace the volume of
oxygen absorbed. Allow the flask to cool to room temperature for about
10–15 minutes then equalize the levels inside and outside the flask, see
the diagram above.
After equalizing the pressure, close the clamp. Place the flask
upright on your bench top and test for oxygen gas in the flask by
plunging a burning splint into it. Results? With your graduated cylinder,
measure the volume of the brown liquid in the flask and in the glass
tube.
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Name
Date
Report for Experiment
Qualitative Observations:
Describe briefly the qualitative evidence that you obtained for the presence
in the air of:
Water vapor
Carbon dioxide
Nitrogen
Quantitative Determination of Oxygen in the Air:
Volume of water required to fill flask (with test tube in it)
mL
Volume of 6M NaOH
mL
Volume of Pyrogallol solution
mL
Volume of all solutions in flask after absorption
mL
Volume of oxygen absorbed
mL
Volume of air originally in the stoppered flask
mL
Percentage of oxygen in air
%
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