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ALUMINUM
USING THE GAS LAWS TO DETERMINE THE % ALUMINUM IN A CAN
Objectives: Use the temperature, pressure and volume of H2 gas to determine the
% of aluminum in a soda can
“Within the course of the last two years .. a treasure has been divined,
unearthed and brought to light ... what do you think of a metal as white as
silver, as unalterable as gold, as easily melted as copper, as tough as iron,
which is malleable, ductile, and with the singular quality of being lighter
that glass? Such a metal does exist and that in considerable quantities on the
surface of the globe." -Charles Dickens
"This valuable metal possesses the whiteness of silver, the indestructibility of
gold, the tenacity of iron, the fusibility of copper, the lightness of glass. It is
easily wrought, is very widely distributed, forming the base of most of the
rocks, is three times lighter than iron, and seems to have been created for the
express purpose of furnishing us with the material for our projectile." -Jules
Verne
SEE: http://www.world-aluminium.org/index.html
In this experiment, you will:
 perform a redox reaction;
 collect a gaseous product;
 determine the % aluminum in a soda can.
First, the chemistry:
“Tin cans” are now made out of aluminum metal, with some impurities.
You can dissolve aluminum metal by the following reaction:
6HCl(aq) + 2Al(s)  3H2(g) + 2AlCl3(aq)
OXIDIZING
AGENTS get
reduced.
REDUCING
AGENTS get
oxidized.
◦
BEFORE YOU COME TO LAB: Assign oxidation numbers to every species in
the above reaction. Identify what gets oxidized and what gets reduced. Identify the
oxidizing agent and the reducing agent. Write the NET IONIC EQUATION for the
reaction.
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Now the gas laws. As you know, if you could measure how many moles of hydrogen gas you
produced in the reaction (6HCl(aq) + 2Al(s)  3H2(g) + 2AlCl3(aq)) you could easily calculate how
many moles of aluminum metal reacted, and thus, how many grams. So far, we’ve been determining
moles of product by getting the mass of the product and then using the molecular weight. Getting
the mass of a gas (H2) is tricky, however. What we’ll do instead is weigh the aluminum can (well, it’s
mostly aluminum), and determine the moles of H2 by measuring the VOLUME, TEMPERATURE
AND PRESSURE of the gas produced. Then, PV=nRT lets us convert to moles. Taking a look at these
measurements one at a time:
VOLUME:
We will measure the volume of H2 generated by WATER DISPLACEMENT. This method is
a bit round-the-barn, but is the most accurate and convenient method for measuring small gas
volumes that SMC has available. How does it work?
Imagine a test tube. How can you find the volume? It’s a featureless glass cylinder! No marking,
nothing. Finding the volume is easy, though. Weigh the empty test tube. Fill it with water. Weigh
the filled tube. If you know the mass of water filling the tube, and the density of water, you can
easily calculate the volume.
Test tube
filled with
water
Say you have a test tube that holds 98.5467 g of water at 25oC. The density of water at
that temperature is 0.997 g/mL. The volume of the test tube is: 98.5467 g (1 mL/0.997 g)
= 98.8843 mL. Simple!
Simple.
Bucket
Water displaced by gas
Now, say you stick your palm over the test
tube, and carefully flip it over into a bucket of
water. The test tube will remain full of water,
even when you talk your hand away. Bubble
some gas into the tube, however, and the gas
will start pushing out, displacing, some of the water. Put your
hand over the mouth of the test tube, flip it back upright and weigh
it again.
Say the test tube that held 98.5467 g of water, which is 98.8843 ml, when full, now
Rubber tube
holds 45.1230 g of water. What’s the volume of the gas? The density of water is 0.997
g/mL. The volume of water still in the test tube is: 45.1230 g (1 mL/0.997 g) = 45.2588 mL.
The volume of gas is 98.8843 ml - 45.2588 mL = 53.6255 mL. Ta Da! An inexpensive, quick and fairly accurate volume
measurement.
Gas generator
TEMPERATURE:
You are going to be collecting H2 gas in a test tube over water, as above. It’s reasonable to
assume that the temperature of the gas will be the temperature of the water. Use a thermometer. Be
sure to remove the protective plastic tip!
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PRESSURE:
OK, so this is a tough one. The chemistry department doesn’t own any gas-collecting
cylinders with attached pressure gauges. What we do have is a barometer which
measures atmospheric pressure in mm Hg. Is this enough? Well, it would be, IF the
pressure of the gas inside the test tube were the same as the atmospheric pressure.
We can make it so. If the water level in the bucket is equal to the water level in the
test tube, then the pressure inside the test tube is the same as the pressure outside,
which is the atmospheric pressure. Life is good.
BEFORE YOU COME TO LAB:
If the water level in the test tube is ABOVE the water level in the bucket, predict whether the
pressure in the tube is higher or lower than atmospheric pressure.
Well, life is pretty good. There’s another little diddle we’ll have to do to figure out pressure of H2 gas
inside the tube. We can easily make the TOTAL pressure in the tube equal to Patm, but the problem is
that H2 isn’t the only gas in the tube. Some water will also go into the gas, or vapor, phase, so the
tube will be filled with a mixture of water vapor and H2. What so we do about the water vapor? (for
historical reasons, H2O(g) is rarely referred to as “gas” but as “vapor”).
In the late 1700’s, John Dalton showed that in mixtures of gases, like ours, each gas exerts a
pressure, known as its partial pressure (or, in the case of water, its vapor pressure) , that is equal to
the pressure the gas would exert if it were the only gas present, and also that the total pressure of the
mixture is the sum of the partial pressures of all the gases present.
PTotal = P1 + P2 + P3 + · · · DALTON”S LAW OF PARTIAL PRESSURES
where PTotal equals the total pressure of the mixture, and P1, P2, P3, . . . . are the partial pressures of the
gases present in the mixture. Both H2O(g) and H2(g) exert a pressure. So
PTotal = PH2 + PH2O. We only care about PH2(g). We know PTotal; that’s Patm. Fortunately, we can look
up PH2O. The warmer the water, the more of it goes into the gas phase, and the higher its vapor
pressure. The handbook of Chemistry and Physics has vapor pressures for water tabulated for
different temperatures. For example, at 25.0 oC, the vapor pressure of water is 23.756 mm Hg.
If Patm is 754.34 mm Hg, (and so is the pressure inside your leveled tube), and it’s 25.0 oC, then the pressure of
H2(g) is: 754.34 mm Hg = PH2 + PH2O = PH2 + 23.756 mm Hg. PH2 = 754.34 mm Hg - 23.756 mm Hg = 730.58
mm Hg. This is referred to as the pressure of “dry” H2.
What is the pressure of “dry” H2 in the above example in ATMOSPHERES?
So, that’s how we will go after measuring temperature, pressure and volume of a gas.
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Before you come to lab: Take another look at the chemical reaction
You will do today:
6HCl(aq) + 2Al(s)  3H2(g) + 2AlCl3(aq).
Calculate the MASS of aluminum you will need to make approximately 35
mL of H2 at STP.
THE EXPERIMENT:
You will need:
200 mm Test tubes for gas generator
175 mm Test tubes for gas collection
Teeny test tube as reaction vessel
Tygon tubing, 1 hole-stopper, glass tubing
Grey plastic bin full of water
50 mL Grad. Cylinder
Ring stands
Clamps
Kimwipes
Alcohol thermometer
Barometer
Glass plates
Plastic taring beakers
1
1
1
1
1
1
2
2
2 boxes/bench
1
Demo Set-up:
Gas-generator/collector on
front bench
1
6 at balances
Aluminum snippets from can < 0.1 g/student pair
Scissors
6 M HCl
50 mL/student pair
Caution: 6M HCl is corrosive! If you get it on your hands, wash thoroughly with water.
Always wear your safety glasses!
THE PROCEDURE:
Before you come to lab: Make an Excel Template of the data chart on
page 7. PUT IT ON A FLOPPY DISK & BRING IT TO LAB.
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1. SET UP YOUR REACTION VESSEL AND GAS-COLLECTING APPARATUS. See the set-up on
the front bench.
A. The largest test tube, the smallest test tube, and the 1-hole rubber stopper with tubing attached
make up your REACTION VESSEL. Clamp the largest test tube to your ring stand. Make sure
the rubber stopper fits. Hold on to the smallest test tube for now.
B. The medium large test tube is your GAS COLLECTING TUBE. You need to fill it completely
full of water.
2. OBTAIN AND RECORD MASSES.
A. Fill the medium large test tube with water, dry the outside thoroughly with a Kimwipe, and
weigh it, using the plastic beakers in the balance area to hold the test tube upright in the balance
case. Be sure you tare the plastic beaker! Record the mass of your test tube & water to the
nearest 0.1 mg. Carefully return to your bench area, and, using the glass plate or Parafilm, clamp
the test tube upside down in the gray plastic bin. The trick of course is to make sure that the test
tube remains full of water. This test tube needs to be COMPLETELY FULL OF WATER, and
you need to know the MASS of the water, since you will use mass of water to calculate volume
of gas collected.
B. Obtain a small piece of aluminum from a soda can. You will need a mass of aluminum that will
generate about 35 mL of H2 at STP. WEIGH the aluminum to the nearest 0.1 mg, record the
mass, and put the aluminum into the smallest test tube, which is part of the reaction vessel.
3. TEMPERATURE .
A. Record the temperature of the water in the gray plastic bin. Be sure you remove the protective
plastic doohicky protecting the tip of the thermometer!
4.
GET READY TO START THE REACTION.
You will generate hydrogen by reacting your aluminum can with 6 M HCl. That’s easy, you just dump the
piece of aluminum into excess HCl & stand back, right? Well, no. The problem is that you want to capture
all the H2 in the gas collection tube, and this reaction starts fast. By the time you jam the stopper back on to
your reaction vessel, half the H2 would be blowing off into the lab. What to do? Is there a way to get all
your reagents ready to mix, get the reaction vessel stoppered, and the Tygon tubing into the gas collector
BEFORE the reaction starts? Sure. Read on….
A. Pour 15 mL of 6 M HCl into your largest test tube. Clamp it onto the ring stand so that it is not
quite parallel to the bench (about 10o of tilt will keep the HCl from pouring right back out). Find
your piece of aluminum (You have previously recorded the mass of the Al snippet in your
notebook, of course). Put the Al snippet into the smallest test tube. Now, carefully slide the
teeny test tube down into your medium large test tube. The teeny test tube will sequester the Al
from the HCl, and all will be quiet. Tightly stopper the largest test tube and run the Tygon
tubing into your water-filled (previously weighed & recorded) medium test tube.
5. START THE REACTION
A. Slowly and carefully tilt the largest test tube, containing the HCl and Al, toward horizontal until
the HCl pours into the smallest test tube. At this point, the reaction will start, and you will see
bubbles of H2 gas collecting in the gas collecting tube. Admire your handiwork until the
bubbling stops.
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6. EQUALIZE PRESSURE IN THE GAS COLLECTING TUBE AND THE ATMOSPHERE.
You will notice that the H2 gas has displaced some of the water in the gas-collecting tube. You now need to
equalize pressure inside & out.
A. Remove the tip of the Tygon tubing from the gas-collecting tube. Now, raise or lower the test
tube until the water levels inside & out of the tube are equal. Now pressures in & out are equal.
Clamp the tube in that position. Slide your glass plate under the tube, unclamp it, and carefully
flip it up to vertical. Be careful you don’t spill any water! Now, dry the outside of the test tube
with a Kimwipe , take it over to the balance area, and weigh it to the nearest 0.1 mg.
7. RECORD atmospheric pressure (it will be written on the board).
8. RECORD water temperature.
9. LOOK UP the vapor pressure of water at your water temperature in the Handbook of Chemistry &
Physics. It will be on the front bench. Record it in your notebook, and also, correctly note the
reference. NO CORRECT REFERENCE, NO LAB.
10. LOOK UP the density of water at your water temperature in the Handbook of Chemistry & Physics. It
will be on the front bench. Record it in your notebook, and also, correctly note the reference. NO
CORRECT REFERENCE, NO LAB
11. REPEAT THE EXPERIMENT 2 more times.
ALL WASTE FROM THIS EXPERIMENT GOES IN THE JUG LABELLED “ALUMINUM
WASTE” IN THE HOOD.
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Dealing with the data:
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You will want to be sure you include the following calculations:
1. Pressure of “Dry” H2 : Use Dalton’s Law, the atmospheric pressure, and the vapor pressure of water.
2. Volume of H2 in mL & L. Use the mass of water displaced and the density of water.
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3. Calculate the volume of “dry” H2 at STP (It’ll be good practice!)
4. As always, calculate the average, standard deviation and % error for your 3 trials.