A Simultaneous Analysis Problem for Advanced General Chemistry Laboratories J. J. Leary' and T. N. Gallaher James Madison University, Harrisonburg, VA 22807 For years the oxidation of magnesium metal in air has been used as an introductory experiment for the determination of the formula of a compound? The experiment herein described employs essentially the same, very simple laboratory procedure but is significantly more advanced in terms of the information sought. The goal of the experiment is to determine the percentage .Mg,N2 and the percentage MgO formed when magnesium metal is ignited in a crucible using a Bnnsen burner. For the past three years this experiment has been successfullv 3-hr laboratorv bv. aw" period . . --" nerformed in a single u proximately twenty students per year in the chemistry majors section of the general chemistrv laboratorv. We think that this is a particularly valuable experiment bkcause the required reasoning process is soundly based on the concept of the mole and because it introduces the concept that it is necessary to have as manv, eouations as there are unknowns. Finally, this . experiment d r r i r11end\.ant.ixt.>that overall ir i-. \.erv s d e and thr clwrnical~iw (I 1\12- ribhun and a>:#rerIitre t ~ w - l u ~ i c . inexpensive, and readily available.
In this experiment the weight percentage of the reaction products, MgO and Mg3N2, is determined. This can he accomplished because reaction (3) converts Mg& to MgO with the evolution of ammonia.
A
Experimental Place a clean dry crucible and cover on a wire triangle. Adjust a Bunsen burner so that its flame is as hot as possible. Heat the crucible and cover, with the cover tilted slightly, until the bottom of the crucible glows a dull red; then continue to heat for an additional 5 min. After heating and for the remainder of the experiment the crucible and cover must not be touched with the hands. Using tongs, place the crucible in a dessicator to caol. Weigh the crucible and cover (throughoutthe experiment all weighings should be recorded to the nearest milligram). It is assumed here that constant weight or nearly constant weieht will be achieved hv this heating.. mocess and that further heating will Drove unnecessarv for the aeeuracv reauired in
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with a stirring rod.3 Place the crucible on a triangle with the cover tilted slightly to allow access of air. Heat slowly at first, then more strongly. If the magnesium starts to hum, and smoke is given off, cover the crucible completely, and remove the flame far a short time. After the contents no lonzer hc;ttinp and w m k ir~ 110 10ndt.rgiv,.u oii, omllnur gkm hriphtlv tt, hcat I ~ P roc , 1 l h strunl:l) i u r 5 i d d ~ r i mi~nll t w t ~ u.i!h ~ I ~ V lvrr C tilled open. >orhott h v Iruttm~..frhc..roi:l~lr.:Il~s;nd!~ll red .\llm the crucible to caol in a dessicator, then weigh. Push the fluffy product down into the bottom of the crucible with a stirring rod. Dust the particles adhering to the rod into the crucible. Add 10 drops of distilled water and note the odor of ammonia. Heat the crucible with the cover in place, gently at first and then to red heat for 5 min; cool in a dessicator, and weigh. Calculations When magnesium metal is burned in air, reactions (1) and (2) take place simultaneously.
Assume that one were to start the exneriment with one mole of magnesium metal (24.31 g) then according to reaction (1) it would be possible to produce 40.31 g of MgO, or via reaction (2) it would be possihle to produce 33.65 g of Mg3N2. This clearly illustrates that a given amount of magnesium will yield a greater mass of MgO than Mg3N2. This further indicates that prior to adding water and heating, the crucible containing MgO and Mg3N2 will weigh less than it weighs following reaction (3). Therefore, the increase in the crucible's weight after reaction (3) is the result of the conversion of Mg3N2to MgO as given by (4) A mass (g) = g MgO (formed in 3) - g MgaNs (reacted in 3) (4) The suhscri~t(formed in 3) will be dropped because from here on we will only he concerned with reaEtion (3). Equations (5) and (6) are then substituted into eqn. (4) giving (7) moles MgO = g MgO/g.m.wt. MgO
(5)
moles M ~ s N= z g MggNz/g.m.wt. MgaNz
(6)
mass (g) = (moles MgO X g.m.wt. MgO) - (moles MgBN2X g.m.wt. Mg3N2) (7)
It is most important to understand the origin of this equation and to realize that it contains two unknown quantities: moles MgO and moles Mg3N2.In all non-trivial cases, it is necessary to have as many equations as there are unknowns before the problem can be solved. The stoichiometry of reaction (3) makes it possible to write a second independent algebraic eqn. (a),containing the same quantities that were unknown in eqn. (7). 3 x males Mg3Nz = moles MgO
Equations (7) and (8) can now be solved simultaneously yielding moles of Mg3N2 (or moles MgO). Subsequently, grams of Mg3N2 can he calculated using eqn. (6). The weight percentage of Mg3N2 (9) is the mass of Mg& just calculated, divided by the total mass of MgO and Mg3N2 prior to reaction (3). wt%Mg8Nz= g Mg~Nz/(gMg3N2 + g MgO) X 100
Author to whom correspondence should be addressed. See for instance "Synthesis and Determination of the Formula of a Compound," David P. Dingledy Willard Grant Press STOI-168 Modular Laboratory Program in Chemistry, 1976. Many students have been surprised that the results desired are obtained without knowing the initial mass of magnesium used.
(8)
(9)
Prior to reaction (3) i t is assumed that only Mg3Nz and MgO are present, the weight % MgO can be obtained simply by using eqn. (10). wt% MgO = (100 - wt% Mg3N2)
Volume 60 Number 8
August 1983
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673
Results and Discussion Data Set Trial Number
lowest individual MgsN2 perc&tages were 18.3 andl.5. The mean MgsN2 concentration for the same semester was 7.0% and the corresponding standard deviation was 5.0%. This experiment is not, and never was, intended to be used as a
this experiment only for advanced groups of students. Even in the advanced lab section where this experiment has been performed, many of the students have difficulty applying these concepts to post-lab quizzes.
Weight of Crucible. Cover Weight of Crucible, Cove6 MgO and
Summary of Data for Fall
1981
A
B
I
I1
I
C Ii
I
I1
19.100 19.924 19.585 19.847 19.255 18.488 19.448 20.326 19.899 20.198 19.591 18.793
b N 2 Weight of Crucible, 19.453 20.333 19.901 20.204 19.596 18.800 Cover, MgO 3.2 8.6 7.5 11.6 7.2 8.8 Weight % of Mg3N2 5.9 9.6 Average Weight % Mg3N2 8.0 90.4 92.0 94.1 Average Weight % MgO
