hypothesis for empirical formula of magnesium oxide

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• Michael Jansen's blog

The Empirical Formula of Magnesium Oxide Lab: A Successful Failure, Next Steps—and an Important Lesson

PART 1: Why this lab is no good

For many years I was troubled by this commonly used, straight-forward, interesting-to-carry-out, and engaging experiment 1 . This analytical, mole concept-based activity can be found in pretty much any chemistry lab manual, from Grade 11 to first-year university.

Procedure-wise, students set out to quantitatively combust a piece of magnesium ribbon in a covered crucible, over a rip-roaring Bunsen burner flame to produce magnesium oxide. A typical apparatus is illustrated in figure 1 2 . During the “cooking”, after the bottom of the crucible becomes red-hot, I visit each student-station, carefully admitting a small quantity of air to the crucible as required for combustion, while simultaneously allowing minimal loss of smoke, which is, after all, magnesium oxide.

burning_mg_chemix.jpg

Figure 1: The apparatus for the determination of the empirical (simplest) formula of magnesium oxide (image created using Chemix ).

Some lab manuals talk about only the reaction of magnesium with oxygen 3 :

Mg(s)  +  O 2 (g)   à   Mg x O y (s)                                                                                                 (1)

More advanced instructions discuss the reaction of magnesium with N 2 (g), which comprises 78% (v/v) of the earth’s atmosphere 4 , to produce magnesium nitride, and the subsequent decomposition of Mg 3 N 2 to give magnesium oxide 5 :

Mg(s)  +  N 2 (g)  +  O 2 (g)  à   MgO(s)  +  Mg 3 N 2 (s)                                                               (2)

MgO(s)  +  Mg 3 N 2 (g)  +  H 2 O(l) à   MgO(s)  +  Mg(OH) 2 (s)  +  NH 3 (g)                                (3)

MgO(s)  +  Mg(OH) 2 (s)  à   Mg x O y (s)  +  H 2 O(g)                                                                  (4)

No matter how I’ve had students analyze their experimental data, results were invariably lack-lustre, with just enough decent values for the simplest formula of magnesium oxide to keep this activity on the roster.

Mg burning in air

Video 1:  Reaction of Magnesium with Oxygen  (derived from Jerrold J Jacobsen and John W. Moore. Chemistry Comes Alive! Vol. 3: Abstract of Special Issue 23 on CD ROM. Journal of Chemical Education 1997 74 (5), p 607-608. DOI: 10.1021/ed074p607), ChemEd Xchange on Vimeo. (accessed 8/12/21)

Several years ago, I was hit by multiple (figurative) lightning bolts:

As part of a pre-lab discussion/demonstration, I burn a piece of magnesium. Students conclude that both the pure white smoke and the pure white ash are magnesium oxide 6 . This shows the importance of not allowing smoke to escape during the combustion. We’re collecting magnesium oxide quantitatively, after all.

a)  The “magnesium oxide” produced in the crucible is not pure white—it is grey 7 . The typical procedure generally notes that the formation of the grey powdery substance signals that the reaction is complete and directs students to cool and weigh the crucible.

b) Magnesium reacts with carbon dioxide to produce carbon and magnesium oxide 8 :

2 Mg(s)  +  CO 2 (g)  à   2 MgO(s)  +  C(s)                                                                             (5)

The Mg in the crucible is bathed in CO 2 (g) from the combustion products of methane, used to fuel the Bunsen burner.

CH 4 (g)  +  2 O 2 (g)  à   CO 2 (g)  +  2 HOH(g) 9                                                                           (6)

This explains the grey product—it’s a mixture of MgO and C.

Reaction of Magnesium with Carbon Dioxide

Video 2:   Reaction of Magnesium with Carbon Dioxide (derived from Jerrold J Jacobsen and John W. Moore. Chemistry Comes Alive! Vol. 3: Abstract of Special Issue 23 on CD ROM. Journal of Chemical Education 1997 74 (5), p 607-608. DOI: 10.1021/ed074p607), ChemEd Xchange on Vimeo. (accessed 8/12/21)

See reaction 5 in the Chemistry Comes Alive #3 video from our ChemEd X video collection above 10 . Reaction (5) is also beautifully demonstrated by Sir Professor Doctor Martyn Poliakoff’s people in the Periodic Table of Videos  clip on Carbon Dioxide (Part II) 11 . The simultaneous production of magnesium oxide and carbon is apparent when magnesium is burned in a dry ice “sandwich” 12 . In a less spectacular, but equally effective demonstration, I simply lower a piece of burning magnesium into a 2-L beaker full of CO 2 (g) 13 . After the reaction, white MgO and black C are apparent.  

As for Mg 3 N 2 , it is a greenish-yellow powder 14 , which I have not, in over 30 years, observed with the naked eye in the reaction products. To be fair, it’s not a vibrant color; perhaps there isn’t enough of it to see.

To sum-up, this activity does not produce what it claims to produce. The empirically observed result—MgO and C—does not line-up with the expected product—pure MgO.

PART 2: Use this experiment as a successful failure

The fact that the “MgO” produced (grey) does not resemble pure MgO (white) is an eye-opener. I’m not proud to say that it took me over 25 years to interpret the plain-as-day empirical evidence. It was, after all, in the lab manual—it had to be true.

In the “Make Lemonade from Lemons” department, I continue to use this experiment—as a successful failure. It is an important teachable moment; I milk it for everything it’s worth.

This is Chemistry, for Pete’s sake—AN EMPIRICAL SCIENCE!!!!!

A favourite post-lab question, which I use as a “soft introduction” 15  to stoichiometry, asks students to calculate the total mass of MgO and C produced when 1.00 g of Mg reacts 50/50 by mass in each of reactions (1) and (5).

PART 3: And then . . .

a) I don’t stop there. I ask students how we could modify this experiment. After some back-and-forth, I reveal a combustion chamber (figure 2) that I built by taping two 2-L beakers together, with the “lips” of the beaker co-incident, to create a small opening.

magnesium_combusion_chamber.png

Figure 2:  The magnesium combustion chamber

I don’t discard the MgO and C from the failed experiment. In a future experiment, I have students gravimetrically analyze the MgO/C mixture. Students can determine the % C in the mixture of MgO and C.

I ignite a weighed piece of Magnesium, which I hold with tongs, and insert it, without delay, into the weighed, empty combustion chamber. If all goes well—inserting the burning magnesium into the opening is tricky—we wait for the magnesium oxide “smoke” to settle before recording the final mass of the combustion chamber.

If I’m not able to accomplish this task after a few attempts, I simple refer my students to the data in Table 1.

Table 1. Reaction of Mg(s)  +  O 2 (g)   à   Mg x O y (s)

reaction_of_mg_with_oxygen.png

In this experiment, it is obvious that the pure white product is indeed magnesium oxide; data support the formula of MgO.

b) I don’t discard the MgO and C from the failed experiment. In a future experiment, I have students gravimetrically analyze the MgO/C mixture. When excess 2.0 mol/L HCl(aq) is added, the MgO reacts as follows:

MgO(s)  +  HCl(aq)  à   MgCl 2 (aq)  +  HOH(l)                                                                          (7)

Mercifully, C does not react, and so after filtering off the aqueous magnesium chloride, and drying and washing, the C collected in the filter paper can be weighed. This can be used to determine the % C in the mixture of MgO and C.

PART 4: Next Steps

After this experiment and the post-lab work, I have students carry out a separate determination of the empirical formula of zinc chloride, according to the reaction:

Zn(s)  +  2 HCl(aq)  à   ZnCl 2 (aq)  +  H 2 (g)                                                                           (8)

In the fumehood, students react a weighed piece of clean zinc with a slight excess of concentrated (12 mol/L) HCl(aq) in a weighed beaker atop a hot plate. The H 2 (g) and unreacted HCl(aq) exit via the chimney. Students simply weigh the dried product, which is ZnCl 2 . Student results support the accepted formula.

This experiment may not be appropriate for your students: At Crescent School we have plenty of fumehood space; students wear lab aprons and nitrile gloves. I’m okay with letting them use a wee bit of 12 mol/L HCl. If that’s not an option for you, a teacher demonstration might be a good idea. 

PART 5: The most important part

It is the zenith of irony when an empirical formula experiment is empirically bogus.

This may sound like it belongs in the “Department of Redundancy Department”, but a post-lab discussion MUST include the essential ingredient of science:  EMPIRICAL EVIDENCE.

Now, more than ever, students—everyone—must be made aware:

Just because an experiment “looks good” or is widely accepted doesn’t make it good.

Just because an experiment “looks good” doesn’t mean it’s carried out properly.

And just because an experiment “looks good” doesn’t mean that it is interpreted properly.

We must teach the importance of asking questions. Questions are more important than answers. I want my students to question everything I do or say. Sometimes—okay, many times—I don’t know the answer; sometimes the explanation will take too long; and sometimes the answer is “because I said so”. The point is—they need to ASK QUESTIONS 16 .

May peace be with you.

• https://uwaterloo.ca/chem13-news-magazine/april-2015/feature/vive-science
• https://edu.rsc.org/experiments/the-change-in-mass-when-magnesium-burns/...
• McGraw-Hill Ryerson chemistry text, 2011
• https://en.wikipedia.org/wiki/Nitrogen
• https://www.webassign.net/question_assets/ucscgencheml1/lab_2/manual.html
• https://en.wikipedia.org/wiki/Magnesium_oxide
• http://www.dynamicscience.com.au/tester/solutions1/chemistry/moleandempi...
• Yehoshua Sivan, “Burning magnesium in a Bunsen blame and other flame experiments”, Chem 13 News, February 2015, pages 12 – 13
• One of my many quirks as a Chemistry teacher is to write the formula for water as HOH, rather than H 2 O, to emphasize the bonding order of the atoms. I do not compel my students to adopt this irregular approach. (Be thankful you don’t live with me . . . )
• https://vimeo.com/419694043
• https://www.youtube.com/watch?v=0dSMzg0UPPo&t=332s
• https://pubs.acs.org/doi/pdf/10.1021/ed055p450.2
• I prepare the CO 2 by reacting HCl(aq) with NaHCO 3 (s)
• https://en.wikipedia.org/wiki/Magnesium_nitride
• I’m a big fan of “foreshadowing” or “soft launching” concepts. See  https://uwaterloo.ca/chem13-news-magazine/february-2017/pedagogy-opinion...
• An upcoming test question will present some information: a table or a graph or something. Students will be required to ask an intelligent question. 

General Safety

For Laboratory Work:  Please refer to the ACS  Guidelines for Chemical Laboratory Safety in Secondary Schools (2016) .  

For Demonstrations: Please refer to the ACS Division of Chemical Education Safety Guidelines for Chemical Demonstrations .

Other Safety resources

RAMP : Recognize hazards; Assess the risks of hazards; Minimize the risks of hazards; Prepare for emergencies

Science Practice: Analyzing and Interpreting Data

Analyzing data in 9–12 builds on K–8 and progresses to introducing more detailed statistical analysis, the comparison of data sets for consistency, and the use of models to generate and analyze data.

Analyzing data in 9–12 builds on K–8 and progresses to introducing more detailed statistical analysis, the comparison of data sets for consistency, and the use of models to generate and analyze data. Analyze data using tools, technologies, and/or models (e.g., computational, mathematical) in order to make valid and reliable scientific claims or determine an optimal design solution.

Science Practice: Asking Questions and Defining Problems

Asking questions and defining problems in grades 9–12 builds from grades K–8 experiences and progresses to formulating, refining, and evaluating empirically testable questions and design problems using models and simulations.

questions that challenge the premise(s) of an argument, the interpretation of a data set, or the suitability of a design.

Scientific questions arise in a variety of ways. They can be driven by curiosity about the world (e.g., Why is the sky blue?). They can be inspired by a model’s or theory’s predictions or by attempts to extend or refine a model or theory (e.g., How does the particle model of matter explain the incompressibility of liquids?). Or they can result from the need to provide better solutions to a problem. For example, the question of why it is impossible to siphon water above a height of 32 feet led Evangelista Torricelli (17th-century inventor of the barometer) to his discoveries about the atmosphere and the identification of a vacuum.

Questions are also important in engineering. Engineers must be able to ask probing questions in order to define an engineering problem. For example, they may ask: What is the need or desire that underlies the problem? What are the criteria (specifications) for a successful solution? What are the constraints? Other questions arise when generating possible solutions: Will this solution meet the design criteria? Can two or more ideas be combined to produce a better solution?

Science Practice: Constructing Explanations and Designing Solutions

Constructing explanations and designing solutions in 9–12 builds on K–8 experiences and progresses to explanations and designs that are supported by multiple and independent student-generated sources of evidence consistent with scientific ideas, principles, and theories.

Constructing explanations and designing solutions in 9–12 builds on K–8 experiences and progresses to explanations and designs that are supported by multiple and independent student-generated sources of evidence consistent with scientific ideas, principles, and theories. Construct and revise an explanation based on valid and reliable evidence obtained from a variety of sources (including students’ own investigations, models, theories, simulations, peer review) and the assumption that theories and laws that describe the natural world operate today as they did in the past and will continue to do so in the future.

Science Practice: Engaging in Argument from Evidence

Science practice: obtaining, evaluating, and communicating information.

Engaging in argument from evidence in 9–12 builds on K–8 experiences and progresses to using appropriate and sufficient evidence and scientific reasoning to defend and critique claims and explanations about natural and designed worlds. Arguments may also come from current scientific or historical episodes in science.

Engaging in argument from evidence in 9–12 builds on K–8 experiences and progresses to using appropriate and sufficient evidence and scientific reasoning to defend and critique claims and explanations about natural and designed worlds. Arguments may also come from current scientific or historical episodes in science. Evaluate the claims, evidence, and reasoning behind currently accepted explanations or solutions to determine the merits of arguments.

Science Practice: Planning and Carrying out Investigations

Planning and carrying out investigations in 9-12 builds on K-8 experiences and progresses to include investigations that provide evidence for and test conceptual, mathematical, physical, and empirical models.

Planning and carrying out investigations in 9-12 builds on K-8 experiences and progresses to include investigations that provide evidence for and test conceptual, mathematical, physical, and empirical models. Plan and conduct an investigation individually and collaboratively to produce data to serve as the basis for evidence, and in the design: decide on types, how much, and accuracy of data needed to produce reliable measurements and consider limitations on the precision of the data (e.g., number of trials, cost, risk, time), and refine the design accordingly.

Science Practice: Using Mathematics and Computational Thinking

Mathematical and computational thinking at the 9–12 level builds on K–8 and progresses to using algebraic thinking and analysis, a range of linear and nonlinear functions including trigonometric functions, exponentials and logarithms, and computational tools for statistical analysis to analyze, represent, and model data. Simple computational simulations are created and used based on mathematical models of basic assumptions. Use mathematical representations of phenomena to support claims.

HS-PS1-2 Chemical Reactions

Students who demonstrate understanding can construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.

*More information about all DCI for HS-PS1 can be found at  https://www.nextgenscience.org/dci-arrangement/hs-ps1-matter-and-its-interactions and further resources at https://www.nextgenscience.org .

Assessment is limited to chemical reactions involving main group elements and combustion reactions.

Examples of chemical reactions could include the reaction of sodium and chlorine, of carbon and oxygen, or of carbon and hydrogen.

HS-PS1-7 Mathematical Representations

Students who demonstrate understanding can use mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction.

Assessment does not include complex chemical reactions.

Emphasis is on using mathematical ideas to communicate the proportional relationships between masses of atoms in the reactants and the products, and the translation of these relationships to the macroscopic scale using the mole as the conversion from the atomic to the macroscopic scale. Emphasis is on assessing students’ use of mathematical thinking and not on memorization and rote application of problem - solving techniques.

All comments must abide by the ChemEd X Comment Policy , are subject to review, and may be edited. Please allow one business day for your comment to be posted, if it is accepted.

Use crown bottle tops and make the experiment work.

Go to https://microchemuk.weebly.com/3-blog-is-this-supposed-to-happen/it-is-n... and see how you can get this experiment to work every time(as reactions should). It is so easy and enjoy collecting the bottle tops. There is an old CLEAPS video here Finding the formula of magnesium oxide . If you have not got the small pipe clay triangles, then place the bottle tops on the gauze section of the gauze, not the ceramic centre. As well as MgO there is Mg₃N₂. To note its presence, place the product, in a vial, add hot water, and put moist red litmus over the top and it goes blue with the ammonia Note I only use 0.12 to 0.2g of magnesium ribbon, coiled around a pencil so it fits in the bottle top sandwich. Cheers Bob Worley in the UK.

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IB Chemistry IA: Determining the Empirical Formula of Magnesium Oxide

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