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.

• 10th Grade
• Chemistry
• 3 activities
• 25 min–40 min

Learning Objectives

1. Calculate the percent composition of each element within a given chemical compound using molar masses.
2. Determine the empirical formula of a compound by converting percent composition data into a whole-number mole ratio.
3. Analyze experimental data to identify an unknown compound based on its calculated percent composition.
4. Compare the molecular formula of a compound to its empirical formula, explaining the mathematical relationship between them.

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.

Prepare & details

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.

Setup: Flexible seating for regrouping

Materials: Expert group reading packets, Note-taking template, Summary graphic organizer

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

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.

Prepare & details

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.

Setup: Standard classroom seating; students turn to a neighbor

Materials: Discussion prompt (projected or printed), Optional: recording sheet for pairs

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

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.

Prepare & details

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.

Setup: Wall space or tables arranged around room perimeter

Materials: Large paper/poster boards, Markers, Sticky notes for feedback

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

Teaching This Topic

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.

What to Expect

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.

Watch Out for These Misconceptions

Common Misconception

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

What to Teach Instead

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.

Common Misconception

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

What to Teach Instead

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.

Common Misconception

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

What to Teach Instead

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.

Assessment Ideas

Quick Check

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

Exit Ticket

After the Think-Pair-Share activity, 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 on an exit ticket.

Discussion Prompt

During the Gallery Walk activity, 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 using examples from the posters.

Extensions & Scaffolding

• Challenge early finishers to research and present a compound with the same empirical formula as glucose but a different molecular formula.
• Scaffolding for struggling students: Provide a guided worksheet with pre-labeled steps for calculating percent composition and empirical formulas.
• Deeper exploration: Have students investigate how percent composition is used in real-world applications like nutrition labels or material science.

Key Vocabulary

Percent Composition	The percentage by mass of each element present in a compound. It is calculated by dividing the total mass of an element in a mole of the compound by the molar mass of the compound and multiplying by 100.
Empirical Formula	The simplest whole-number ratio of atoms of each element present in a compound. It represents the relative number of atoms, not the actual number.
Molar Mass	The mass of one mole of a substance, typically expressed in grams per mole (g/mol). It is calculated by summing the atomic masses of all atoms in a chemical formula.
Mole Ratio	The relative number of moles of reactants and products in a chemical reaction, or the relative number of atoms of elements in a compound, expressed as a ratio.