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I-08: Percent composition and chemical formulas

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Module I-08 · Recommended before: Before empirical formulas and hydrates

Percent composition and chemical formulas

Formula composition connects component masses to a whole sample. Use mole ratios rather than mass ratios to determine empirical formulas.

Learn the method

mass percent = (mass of element/mass of compound) × 100

  1. Convert element masses into moles.
  2. Divide each mole amount by the smallest.
  3. Multiply ratios to obtain small whole numbers when needed.
  4. Verify the final formula and compute percent composition.

Common mistake: Never determine subscripts directly from grams; convert masses to moles first.

Worked example

Problem: A compound contains 12.01 g C and 32.00 g O. Determine its empirical formula.

Step-by-step solution: 12.01 g C ÷ 12.01 g/mol = 1.00 mol C; 32.00 g O ÷ 16.00 g/mol = 2.00 mol O. Empirical formula CO₂.

Check the units and whether the answer is reasonable before continuing.

More applications of this mathematics

Try each extension before expanding its solution.

Empirical formulas from masses

12.01 g carbon and 32.00 g oxygen correspond to 1.00 mol C and 2.00 mol O, producing CO₂.

Hydrates

If 2.00 g anhydrous salt corresponds to 0.0200 mol formula units, and 1.80 g water is lost (0.100 mol), hydrate ratio water:salt = 5:1.

Molecular formula multiplier

An empirical formula CH₂O has empirical formula mass 30.03 g/mol. If the molar mass is 180.18 g/mol, multiplier=6 and formula=C₆H₁₂O₆.

Independent practice — five questions

Suggested mastery threshold: four out of five. Incorrect answers display a worked explanation after submission.

Choose one response for each question, then select “Check my answers.” No timer. Hints are optional. You can retry freely. Questions are formative, not proctored.

Question 1. A 24 g compound sample contains 6 g of oxygen. What is oxygen mass percent?
Hint for question 1

Use mass of element over total compound mass.

Question 2. A 28 g compound sample contains 7 g of oxygen. What is oxygen mass percent?
Hint for question 2

Use mass of element over total compound mass.

Question 3. A 40 g compound sample contains 8 g of oxygen. What is oxygen mass percent?
Hint for question 3

Use mass of element over total compound mass.

Question 4. A 45 g compound sample contains 9 g of oxygen. What is oxygen mass percent?
Hint for question 4

Use mass of element over total compound mass.

Question 5. A 50 g compound sample contains 10 g of oxygen. What is oxygen mass percent?
Hint for question 5

Use mass of element over total compound mass.

Supporting reference

OpenStax Chemistry 2e / NIST — reference for this subject. This lesson’s wording, examples and questions are original.

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