Chemistry · 10th Grade

Active learning ideas

Percent Composition and Empirical Formulas

Active learning helps students connect abstract chemistry concepts to real-world understanding. Calculating percent composition and empirical formulas requires students to move between formulas, masses, and ratios, which hands-on activities make concrete.

• Common Core State Standards
• STD.HS-PS1-7
• STD.CCSS.MATH.CONTENT.HSA.CED.A.1

• 25–40 min
• Pairs → Whole Class
• 3 activities

Activity 01

• Jigsaw
• 40 min · Small Groups

Jigsaw: Four-Step Empirical Formula

Each student in a group masters one of the four steps (grams, moles, divide by smallest, write formula). Groups rotate so each expert teaches the others their step. The class then attempts a new unknown compound together, with each expert verifying their step as the group reaches it.

Calculate the percent composition of elements in a compound.

Facilitation Tip

For the Jigsaw activity, assign each group one step of the four-step empirical formula process and have them teach their step to the rest of the class.

What to look for

Present students with the chemical formula for glucose (C6H12O6). Ask them to calculate the percent composition of carbon, hydrogen, and oxygen in glucose. Review answers as a class, focusing on common calculation errors.

• Understand
• Analyze
• Evaluate
• Relationship Skills
• Self-Management

Activity 02

• Think-Pair-Share
• 25 min · Pairs

Think-Pair-Share: Forensic Unknown

Students receive fictional lab data (percent composition of a white powder: Na, C, O) and individually determine the empirical formula. Partners compare their work and identify where results diverged. The class discusses how this approach is used in forensic chemistry to distinguish between compounds like NaHCO₃ and Na₂CO₃.

Determine the empirical formula of a compound from experimental data.

Facilitation Tip

During the Think-Pair-Share activity, provide forensic unknown data on index cards so students must collaborate to interpret and solve the problem.

What to look for

Provide students with a hypothetical compound that is 40.0% Carbon, 6.7% Hydrogen, and 53.3% Oxygen by mass. Ask them to determine the empirical formula of this compound and write down the steps they followed.

• Understand
• Apply
• Analyze
• Self-Awareness
• Relationship Skills

Activity 03

• Gallery Walk
• 30 min · Pairs

Gallery Walk: Comparing Real Compounds

Stations pair compounds that share similar empirical formulas (CH₂O covers glucose, acetic acid, formaldehyde) so students see how one empirical formula can represent very different substances. Students answer guided questions at each station about what the empirical formula can and cannot tell you about a compound.

Analyze how forensic scientists use percent composition to identify unknown powders.

Facilitation Tip

For the Gallery Walk, prepare posters with real compounds like water (H2O) and hydrogen peroxide (H2O2) so students can compare and contrast mass ratios visually.

What to look for

Pose the question: 'Why is the empirical formula considered the 'simplest' ratio, and how does it relate to the molecular formula?' Facilitate a class discussion where students explain the difference and the mathematical conversion between the two.

• Understand
• Apply
• Analyze
• Create
• Relationship Skills
• Social Awareness

A few notes on teaching this unit

Teachers should emphasize the step-by-step process of moving from formula to mass to ratio, using labeled examples to prevent confusion between atomic mass, subscripts, and molar mass. Avoid rushing through worked examples; let students articulate each step in their own words. Research suggests that students grasp these concepts faster when they repeatedly apply the process to varied examples in small groups.

By the end of these activities, students should confidently calculate percent composition from a formula and determine empirical formulas from mass data. They will also explain why empirical formulas represent simplest ratios and how they differ from molecular formulas.

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Watch Out for These Misconceptions

• During the Jigsaw activity, watch for students assuming the empirical formula is identical to the molecular formula.

  Use the glucose example in the Jigsaw activity to show that C6H12O6 simplifies to CH2O, and have groups compare their molecular formula samples to the empirical formula to reinforce the difference.

• During the Think-Pair-Share activity, watch for students rounding mole ratios to whole numbers prematurely.

  During the Think-Pair-Share activity, provide intentionally messy data (e.g., ratios like 1.5 or 1.33) and have students justify their rounding decisions in pairs before sharing with the class.

• During the Gallery Walk, watch for students dividing by the number of atoms instead of using molar mass for percent composition.

  During the Gallery Walk, post labeled worked examples next to each compound showing how to calculate total mass contribution per element before dividing by the molar mass.

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Methods used in this brief

• Jigsaw
• Think-Pair-Share
• Gallery Walk