Chemistry 211 Lab
Fall 2002
Lab #6 -- Practical Use of the Ideal Gas Law:
Identification of an Unknown Metal Sample
Purpose
The identification of a metal sample, chosen at random from three possibilities. The sample will
be reacted with aqueous strong acid, which will result in the production of a gas. The gas sample is
collected over water, and then quantified by applying the Ideal Gas Law. Comparison of reaction
stoichiometry is then used to determine moles of unknown metal, and with the mass of the unknown
sample, molar mass is obtained (g/mol), providing a basis for the identification of the unknown metal.
Introduction
The Ideal Gas Law is a staple of modern chemistry textbooks, and it inspires much fear and
loathing amongst undergraduate students. One can speculate that the reason for this is the rote
calculation involved in using the law, and not necessarily the law itself. The Ideal Gas Law takes
advantage of a special set of physical conditions, called collectively standard temperature and
pressure, or "STP" for short. Standard temperature is 273.15 K and standard pressure is 101,325 Pa.
Using SI units, one mole of any gas sample (regardless of formula or mass) collected at STP
occupies a equally standard volume in liters. The modern, textbook version of the Ideal Gas Law is the
result of a combination of several other laws concerning the properties of gases:
Boyle's Law:
P x V = constant (at constant temperature, sample size), or
pressure and volume are inversely proportional
Charles' Law:
V = constant x T (at constant pressure, sample size), or
volume and temperature are directly proportional
Avogadro's Law:
V  n (at constant temperature, pressure), or
volume is proportional to the number of gas molecules
There is no direct, mathematically valid way to combine these laws into what is today called the Ideal
Gas Law; but you should take a moment to notice how Boyle's Law, Charles' Law, and Avogadro's Law
can be equally well expressed using the Ideal Gas Law.
Modern Ideal Gas Law:
P V = n R T (valid at 'ordinary' pressures and temperatures), where
P = pressure, V = volume, T = temperature, n = moles of
gas, and R = the Ideal Gas Law constant.
It is customary to measure these various quantities in SI units, or to convert measured values into SI
units from other unit systems. When SI units are used*, R has the value 0.08206 L • atm / mol • K.
* NOTE: the SI unit of pressure is the Pascal (Pa). It is common, however, to use atmospheres (atm) as the pressure unit for
gas law calculations, thereby requiring the memorization of only one value for R (1 atm = 101,325 Pa = 760 torr).
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Design of the Experiment and Background Chemistry
A sample of an unknown metal will be allowed to react with aqueous hydrochloric acid (HCl).
Hydrogen gas will be produced as one of the reaction products, and a water-soluble metal chloride salt
will be the other product:
Unknown Metal #1 and #2:
M(s) + 2HCl(aq) ----------> MCl2(aq) + H2(g)
Unknown Metal #3:
2M(s) + 6HCl(aq) ----------> 2MCl3(aq) + 3H2(g)
The unknown metal will dissolve as the reaction proceeds, and the gas produced will be collected over
water and quantified by volume (mL). Using the Ideal Gas Law, the value n, number of moles of gas
produced, can be determined, assuming the other variables are also measured. Take a moment to rearrange the Ideal Gas Law to solve for n.
You will measure the atmospheric pressure in the lab the day you do the experiment (or, have
that value provided by the Teaching Assistant or prep. room staff). It is unlikely they will be using a
barometer that measures pressure in atmospheres, so expect to convert the recorded pressure to
atmospheres from some other pressure unit. Your textbook should have the necessary tables to convert
pressure to atmospheres from other commonly used pressure units.
Volume measurements will be in milliliters (mL). Again, this will need to be converted to a
volume unit friendly to the value for R given in this text. Likewise, temperatures will need to be
converted to Kelvin from degrees Celsius.
The objective, after each of three trials, will be to solve for n, moles of gas produced. Using the
above stoichiometric relationships, the number of moles of metal consumed can be determined.
Dividing grams of metal used by moles of metal gives the molar mass (g/mol) for the unknown metal.
One last thing to keep in mind: the gas being collected is not just hydrogen. Since it is being
collected over water, the pressure of just the hydrogen gas inside the collection tube needs to be
corrected for the presence of water vapor. Thus, the total pressure of the gas collected is dictated by
Dalton's Law of Partial Pressures:
Ptot = P (H2) + P (H2O)
The vapor pressure of water needs to be subtracted from the total pressure. It is the pressure due to
hydrogen gas alone that must be used to determine n, using the Ideal Gas Law. The vapor pressure of
water at various temperatures can be found in a variety of reference books, including the 211 lecture
text. Values not precisely stated (e.g., the 23.1°C in the sample data) in such tables can be reasonably
interpolated from the two available values above and below the determined temperature of the collected
water vapor:
Vapor pressure of water, 23°C:
21.07 torr
Vapor pressure of water, 24°C:
22.38 torr
Difference:
1.31 torr
Difference x (0.1):
0.131 torr
Interpolated vapor pressure of water (v.p. @ 23°C + difference x (0.1)):
2
21.07 + 0.131 = 21.20 torr
Sample Data and Calculations
Sample of an unknown metal, following #1 or #2 stoichiometry:
Volume of gas produced:
Temperature: 23.1°C (296.3 K)
Atmospheric Pressure: 761.4 torr
Interpolated Vapor Pressure of Water @ 23.1°C: 21.20 torr
0.111 g
43.6 mL (0.0436 L)
Corrected pressure of gas sample: 761.4 torr - 21.2 torr = 740.2 torr
Conversion of torr to atm: 740.2 torr x (1 atm/760 torr) = 0.9755 atm
Solving for n: P V = n R T
(0.9755 atm) (0.0436 L) = n (0.08206 L atm/mol K) (296.3 K)
(0.0425 atm L) = n (24.31 L atm/mol)
n = 0.00175 mol H2
For Unknown Metal of type #1, moles H2 = moles of unknown metal: 0.00175 moles unknown metal
Divide the mass of unknown metal by the stoichiometric equivalent moles of unknown metal:
(0.111 g) / (0.00175 mol) = 63.4 g/mol
Consulting a periodic table of the elements, you would be safe in concluding that the unknown metal is
most likely copper (Cu). Copper's molar mass is 63.546 g/mol. It is not necessary to correctly identify
the unknown metal, provided your calculations lead you to a reasonable conclusion of the unknown
metal's identity.
Note that copper metal, which has a distinctive reddish colour, will not be provided as an unknown
sample in this experiment. Neither will any Group IA metal be among the unknowns. However, any
other stable element of the first transition series may be one of the unknowns, as well as common, nonpoisonous metals from other parts of the periodic table.
You will perform 3 trials of the full experimental procedure for your assigned unknown
metal. For each trial, you will determine n, relate this value for n to moles of unknown metal using the
given stoichiometric relationships, and determine the mass of unknown metal-to-moles of unknown
metal ratio. You will then average the three mass/moles ratios before attempting to identify your
unknown metal sample.
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Experimental Procedure
Equipment:
Chemical Reagents:
1 L beaker
medium test tube (at least 6" in length)
50 mL graduated cylinder
ring stand and utility clamp
one-hole stopper w/
glass tubing gas outlet
rubber tubing, about 12 inches in length
1 piece weighing paper
250 mL beaker (to hold reaction tube)
unknown metal sample, pre-cut
(Teaching Assistant will provide you an unknown
number, use this unknown for all three trials)
10 mL 6 M HCl (enough for all three trials)
Begin by filling the 1 L beaker with tap water, to about the 900 mL mark. Next, fill the 50 mL
graduated cylinder with water, as completely as possible, and place the piece of weighing paper over the
mouth of the cylinder. Then, taking care to lose as little water as possible (some loss is to be expected,
and what to do to correct for this loss will be explained shortly) invert the cylinder into the 1 L beaker.
This is best accomplished by holding the weighing paper in place with one hand, while quickly picking
up and inverting the cylinder with the other hand. Remove the weighing paper after the cylinder has
been inverted and placed into the 1 L beaker. Temporarily clamp the cylinder in place in this inverted
position.
Continue construction of the apparatus by threading the rubber tubing of the pre-assembled stopper/gas
delivery system under the water line, and into the mouth of the inverted graduated cylinder. It should be
far enough into the cylinder to ensure that it will not come out while gas is being collected. Place the
medium test tube into a 250 mL beaker after it has had placed in it a 10 mL portion of 6 M HCl solution.
Test fit the stopper onto the top of the test tube, being careful not to allow the open end of the rubber gas
delivery tube to slip out of the graduated cylinder--the fit should be snug to prevent gas escaping from
around the stopper/gas delivery assembly during the reaction. See Figure 1 for a picture of the
completed assembly.
At this point, with the assembly complete, take a moment to have it inspected by your Teaching
Assistant. You can also get your unknown metal assignment at this time, if it has not already been given
to you. Obtain a sample of your unknown metal, and measure the mass of the sample to three decimal
places. Prepare the metal sample for reaction by carefully rolling or folding it, so that it will drop into
the test tube containing the acid without becoming hung up on the side of the test tube.
Obtain readings for the temperature of the room (°C) and the atmospheric pressure (most likely torr or
inHg). Convert the temperature and pressure measurements to units friendly to the use of the R-constant
0.08206 L atm / mol K.
If some air was allowed into the graduated cylinder in the process of inverting it into the 1 L beaker,
take a reading of this initial volume of trapped air (V(initial)). Keep in mind that you are reading the
cylinder upside down when you do this. This initial volume should be subtracted out after the gas
collection is complete by V(final) - V(initial).
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To begin gas collection, loosen and remove the stopper on the test tube containing the acid. Carefully
but quickly drop the metal sample into the acid solution and replace the stopper immediately, making
sure the fit is snug. Gas evolution commences at once. It may take several minutes to completely
dissolve the metal sample, but you must wait until the metal sample completely dissolves before
continuing. It may be necessary to raise or lower the graduated cylinder as gas is evolved, so watch the
apparatus carefully, and make the proper adjustments if need be.
When the metal sample has completely dissolved, and no more gas can be seen evolving from the test
tube or gas delivery tube, carefully remove the delivery tube from the mouth of the graduated cylinder.
The level of the water inside the graduated cylinder will either be somewhat above or below the level of
the water in the large beaker. Before taking a volume reading, carefully loosen the clamp holding the
graduated cylinder and by hand raise or lower the cylinder until the water level inside and outside the
cylinder is the same. Take a volume reading, keeping in mind (again) that you are reading the cylinder
upside down. This is V(final).
To obtain further trials, it is only necessary to refill with water, and re-invert, the graduated cylinder, and
to replace the gas delivery tube into the mouth of the graduated cylinder. The same sample of 6 M HCl
is still strong enough to dissolve several more metal samples of the size you'll be using. Do at least three
trials with the unknown metal you have been assigned, and use the data collected to ultimately make a
reasonable guess as to the identity of the unknown metal.
Waste Disposal Information
When you have finished your three trials, pour the remaining acid from the medium test tube into the
water contained in the 1 L beaker. Stir the resulting solution to completely dilute the acid, before
flushing the entire contents down the sink with running water.
There should be no unreacted pieces of unknown metal to content with; but if any such pieces remain in
the test tube, dissolve them with a few mL's of the 6 M HCl solution before diluting and rinsing the
contents of the test tube (now a clear, metal fragment-free solution) down the drain.
Return the reaction vessel/gas collection tube and plumbing, clean and shaken dry, to the Teaching
Assistant or the prep. room staff.
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Data Tables and Calculated Quantities
(note: values with an asterisk (*) are values for which you should show at least one completely worked
example in your lab report).
Number of the unknown metal assigned to you:
_____
Trial #1
Trial #2
Trial #3
Mass of unknown metal sample:
__________
__________
__________
Initial volume of trapped air (if any):
__________
__________
__________
Final volume (evolved H2, trapped air):
__________
__________
__________
Total volume of H2 produced:
__________
__________
__________
Room temperature (°C):
__________
*K: __________
Room atmospheric pressure (given in lab): __________
unit: __________
*Room atmospheric pressure (atm)
__________
*Vapor pressure of water, interpolated for room temperature:
__________
*Pressure of H2 gas:
__________
*Moles of H2 gas evolved
__________
__________
__________
*Molar equivalent of unknown metal:
__________
__________
__________
*Mass metal-to-moles metal ratio:
__________
__________
__________
*Average mass-to-moles ratio:
__________
Based upon the value for average mass-to-moles ratio, I conclude that my unknown metal is:
______________________
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Further Questions for Thought and Review
(include detailed answers, with calculations when appropriate, in your lab report)
1). Explain why it is necessary to bring the water level inside the graduated cylinder equal to the water
level outside the graduated cylinder, after gas collection is complete. Which of the Ideal Gas Law's
variables are likely to be affected if you don't (and how)?
2). If calcium metal was used as an unknown in an experiment of this type (and it may!), what would be
a reasonable value for 'n', assuming the sample mass was 0.063 g?
Write the balanced chemical equation for the reaction of calcium metal with aqueous hydrochloric acid.
Using the value for n determined above, determine the amount of hydrogen gas produced, in mL.
Assume temperature and pressure to be the same as they were the day you did the experiment in lab.
3). Explain how failing to account for any trapped air inside the graduated cylinder at the start of the
experiment affects the calculation of:
Volume of H2 gas produced
Number of moles of H2 gas
Molar mass of the unknown metal sample
4). What changes (if any) would you make to the experimental procedure if you decided to use aqueous
sulfuric acid (H2SO4) instead of aqueous hydrochloric acid (HCl)?
Write a balanced chemical equation for the reaction of the unknown metal you were given with aqueous
sulfuric acid.
5). Explain how the collection and quantification of CO2 gas, via the following balanced chemical
reaction, could be used in a method to determine the purity of sodium carbonate.
Na2CO3(s) + 2HCl(aq) --------> 2NaCl(aq) + H2O(l) + CO2(g)
6). What are the units of temperature and pressure commonly used in daily weather reports from media
outlets in:
Washington, D.C.
Toronto, Canada
London, U.K.
T: _____ P: _____
T: _____ P: _____
T: _____ P: _____
Are any of these locations using SI units? Suggest a reason that weather records are not kept in SI units.
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