3 of 20

Notes for Teachers

List of Materials

Iron Metal Strips - 2 strips per group

1 oxidized, the other not.

Iron Oxide - Recommended to chemically rust the metal above so the samples are similar and comparable in size and mass.
Crucible and Lid - Stainless Steel or Porcelain work, but recommend steel
Ring Stand (Support Stand) and Ring (Ring Clamp)
Clay Triangle sized for the crucible
Crucible Tongs
Bunsen Burner
Striker
Scoopula
OMG! Lab Handout
Practice Worksheet

Context: This lesson is expected to take 3-4 days for the full lesson to complete.
Prior to this lesson, students should know what a synthesis reaction is and the Law of Conservation of Mass.
Students will begin by observing the difference between pure iron and iron oxide and investigating what iron oxide is and how iron oxide is a synthesis of iron and oxygen.
After that, students will conduct a pre-lab to familiarize themselves with the lab protocol.
After the lab, the students will conduct a post-lab analysis.
Lastly, the teacher will lead students to understanding how this equates to an Empirical Formula with post-lab calculations.

4 of 20

Standards

Essential HS.P1U1.1

Develop and use models to explain the relationship of the structure of atoms to patterns and properties observed within the Periodic Table and describe how these models are revised with new evidence.

Plus HS+C.P1U1.7

Use mathematics and computational thinking to determine stoichiometric relationships between reactants and products in chemical reactions.

Mathematical Practices:

P.MP.4 Model with mathematics.

P.MP.6 Attend to precision.

P.MP.7 Look for and make use of structure.

Science, Math and Engineering Practices

plan and carry out investigations
analyze and interpret data
use mathematics and computational thinking
construct explanations and design solutions
engage in argument from evidence
obtain, evaluate, and communicate information

Core Ideas for Science

U1: Scientists explain phenomena using evidence obtained from observations and or scientific investigations. Evidence may lead to developing models and or theories to make sense of phenomena. As new evidence is discovered, models and theories can be revised.

U2: The knowledge produced by science is used in engineering and technologies to solve problems and/or create products.

5 of 20

National Standards

Next Generation Science Standards (NGSS)

HS-PS1-7: Use mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction.

Connection: Students calculate the mass of oxygen added to magnesium during a synthesis reaction (conservation of mass) and use these values to determine the stoichiometric ratio (empirical formula) of the product.

HS-PS1-2: 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.

Connection: Students investigate the synthesis of magnesium and oxygen to form magnesium oxide, observing how elements combine in specific whole-number ratios based on their atomic properties.

Common Core Mathematics (CCSS.MATH)

HSN.Q.A.1: Use units as a way to understand problems and to guide the solution of multi-step problems; choose and interpret units consistently in formulas.

Connection: Students use dimensional analysis (stoichiometry) to convert measured masses (grams) into moles and then into simple whole-number ratios.

MP6: Attend to precision.

Connection: Students must measure masses carefully using a digital scale (e.g., to two decimal places) and maintain precision during calculations to ensure the correct empirical formula is derived.

Common Core English Language Arts (CCSS.ELA)

RST.11-12.3: Follow precisely a complex multistep procedure when carrying out experiments, taking measurements, or performing technical tasks; analyze the specific results based on explanations in the text.

Connection: Students follow a specific lab protocol (heating crucible, cooling, reheating) to ensure the reaction goes to completion and accurate data is collected.

WHST.11-12.9: Draw evidence from informational texts to support analysis, reflection, and research.

Connection: In the post-lab analysis, students use their experimental data to calculate the empirical formula and justify their results based on chemical principles.

6 of 20

Objectives:

Use conversion factors to calculate the moles of reactants to obtain the mole relationship within the products and calculate the empirical formula of a compound.
Use mass by difference to understand the mass of the product and amount of gaseous reactant used in a reaction.
Identify a synthesis reaction and apply knowledge of synthesis to calculate the amount of gaseous oxygen used in a reaction.
Use the Law of Conservation of Mass to understand the mass relationship of a synthesis reaction to apply calculations of reactants and products.

7 of 20

Agenda

Day 1:

Have samples of the iron metal ready to go. Students should have one that has not rusted and one that you chemically rust. The pieces should be close in mass before you rust the metal sample.
Students should observe how the two samples compare and take observations of the phenomena of rusting. (10-15 minutes)
After taking observations, allow students to take measurements of what they see and experiment with the iron taking mass, testing the iron, visual data, etc. The depth of this depends on where you are in the curriculum with reactions. (15-20 minutes)

After they have taken observations, facilitate a discussion about what rusting is and what type of reaction it is, aka synthesis.
Discuss where the oxygen for this reaction is coming from and what would happen if all of the iron was to rust, what product would that produce?
Review the Law of Conservation of Mass and how this relates to what they are seeing. (20 minutes)

8 of 20

Agenda

Day 2:

Pass out the OMG! Lab Handout and have students read through the procedures. (5-10 minutes)
Students should complete a pre-lab in the manner that the classroom teacher has set up. (10-15 minutes)

Suggested to write out the procedures and setup a data table in their notebooks that they use in the lab.

Discuss the key safety and clear up any procedural questions students may have (10 minutes)
Have the students wear safety gear and practice moving the crucible and lids with the crucible tongs. This takes practice not to drop and if you are using the porcelain ones you want practice! (15-20 minutes)

Day 3:

Have students complete the lab!
The lab itself can take from 25-40 minutes depending on the students.
When they finish the lab, students can start looking at the analysis and how to analyze the data.
Assign analysis questions #1-6 for them to complete by the next class period.

9 of 20

Agenda

Day 4:

Students should have the lab analysis completed for their data through question #6.
The teacher will facilitate a discussion and teach how to calculate the empirical formula with sample calculations and sample data.
Students will finish the lab and report depending on how the teacher does lab reports.
Students can then use the practice “Empirical Formulas” Worksheet to continue practice with this concept.

10 of 20

What is an empirical formula and how do we calculate it?

Students take observations and data collection from two samples, as sample of pure iron and a sample that has been rusted. They apply the Law of Conservation of Mass and their understanding of synthesis reactions to calculate the Empirical Formula of the oxidation of another compound.

11 of 20

Initial Phenomena and Activity

Students will work in groups of 3-4 to take observations on two samples of iron, one rusted and the other not.
Students will collect data to understand what is happening between the two samples and how it relates to the synthesis.
Students can experiment with the samples including taking mass, scraping off pieces (up to the teacher), testing it with an indicator solution, etc. depending on where the students are in the curriculum.
Afterward, the teacher will facilitate the students through a discussion of what they saw, what a synthesis reaction is, what oxidation is, and where the oxygen comes from in this reaction.

12 of 20

Hands-on Activity Instructions

Students will work in lab groups based on the teacher’s structure in the lab to complete the lab procedures below.

Procedure:

Obtain your crucible and lid and take to your lab bench.
Set up your ring stand with a ring and clay triangle at a height that will allow the bunsen burner’s flame to appropriately touch the crucible.
Clean the crucible by scraping the inside the best you can to remove any excess particles.
Weigh and record the mass of the crucible.
Next, obtain 6 inches magnesium ribbon and coil the ribbon as demonstrated.
Weigh and record the mass of the magnesium.
Then, weigh and record the mass of the magnesium ribbon and crucible to ±0.01 g.
Place the crucible securely on the clay triangle. Set the lid slightly off-center on the crucible to allow air to enter but to prevent the magnesium oxide from escaping.

13 of 20

Hands-on Activity Instructions - Cont.

Procedure:

9. Light the Bunsen burner and brush the bottom of the crucible with the flame for about 1 minute; then, place the burner under the crucible and heat with the highest direct heat.

10. Heat until all the magnesium turns into gray-white powder (probably around 15 minutes). Make sure to lift the lid every 30 seconds to a minute to allow air to flow into the crucible.

11. Stop heating and allow the crucible, lid and contents to cool on the base of the ring stand for 5 minutes. Move it around CAREFULLY on the ring stand base to distribute the heat.

12. Once cooled, weigh and record the mass of the crucible and product to ±0.01 g.

13. Throw away product in designated waste beaker. Scrape any extra product out using a scoopula as best you can. Return the crucible and lid.

Above - Still reacting - look for embers and flaring up.

Below - No longer reacting - Crucible is glowing but the Mg has stopped flaring up.

14 of 20

What is an Empirical Formula?

Mg + O₂ → Mg_?O_?

Empirical Formula = is the simplest whole number ratio of atoms of each element in a compound. Or, in simple words, it is the simplest chemical formula of a compound.

15 of 20

Calculating Empirical Formula Steps:

Convert to mole form
Find mole ratio by dividing each element by the element with the smallest # of moles
If needed, multiply by some integer to get whole numbers (ALL ELEMENTS).

16 of 20

Calculating Empirical Formula Steps, Sample:

NOTE: The #’s used below are made up to NOT give the answer, but to show how to to the analysis and calculations for Empirical Formulas.

The initial mass of Magnesium was 2.43g as shown on the scale in the lab.�
Mg & O product in crucible - crucible = Mg & O Product�20.48g – 14.05g = 6.43 g Mg & O Product�
Mg &O – Mg = O mass gained�6.43 g - 2.43g = 4.00 g Oxygen�
� = 0.09996 mole Mg

2.43 g Mg	1 mole Mg
	24.31 g Mg

17 of 20

Calculating Empirical Formula Steps, Sample:

5. � = 0.25 mol O

6. 0.09996 mols O: 0.25 mols O

7. � = 1 mol O = 2.5 mol Mg

1 x 2 = 2 2.5 x 2 = 5 Mole ratio = 2 mol Mg: 5 mol O

4.0 g O	1 mole O
	16.0 g O

0.09996 mol Mg
0.09996

0.25 mol O
0.09996

18 of 20

Calculating Empirical Formula Steps, Sample:

8. The Empirical Formula for this data is Mg₂O₅

9. This is up to each student to ID

From the lab, the correct ratio is MgO with a 1:1 mole ratio.

19 of 20

Assessment

Class discussion
Formative Assessment Practice Problems - within the sample data
Empirical Formulas Worksheet
Formal Lab Report - Analysis Questions and Conclusion

20 of 20

Differentiation

Strategically pair students as needed.

Provide key formula and vocabulary with images.

Provide additional book and online resources.

Remediation

Extension/Enrichment

For students considering AP^Ⓡ Chemistry or in Honors Chemistry, students should take this further and use molecular mass data to identify the Molecular Formula of a substance.

Extensions for this is the Molecular Determination of Empirical Formulas from Percent Composition.