Empirical and Molecular Formulas from Composition · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

From weighings to formulas

Science · Matter & Materials · ages 15-16
Name ______________________   Date ____________

  1. A compound is 40 percent carbon. How do you start?

    • Pretend 100 grams, so 40 grams of carbon
    • Divide 40 by 100 twice
    • Treat 40 as the subscript

  2. From measured masses, what is the first step toward a formula?

    • Divide each mass by its atomic mass
    • Add all the masses together
    • Guess subscripts that look right

  3. An empirical formula shows the actual atom count in one molecule.

    Circle one:   True   False

  4. An empirical formula of HO has mass 17. The molar mass is 34. What is the multiplier n?

    Answer: ______________

  5. 24 grams of carbon and 4 grams of hydrogen give what empirical formula?

    • CH3
    • C2H
    • CH2

  6. Hydrogen peroxide is H2O2. What is its empirical formula?

    • H2O
    • HO
    • HO2

  7. A student finds CH and declares the compound must be acetylene. Why is that wrong?

    • CH proves nothing can be known
    • Benzene shares CH yet is a different molecule
    • Acetylene has no empirical formula

  8. A student gets a ratio of 1 to 1.5 and writes C1H1.5. What should they do?

    • Round 1.5 down to 1
    • Keep the decimal in the formula
    • Multiply both numbers by 2
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Answer key

For grown-ups. Fold this page away before handing over the rest.

From weighings to formulas W1-mt_4rN5MQaTf7-s1

  1. Pretend 100 grams, so 40 grams of carbon · A 100 gram sample turns each percent into grams.
  2. Divide each mass by its atomic mass · Masses become moles first, and moles show the ratio.
  3. False · It shows only the simplest ratio, not the true count.
  4. 2 · Divide 34 by 17 to scale the ratio up.
  5. CH2 · The moles are 2 and 4, so the ratio is 1 to 2.
  6. HO · The 2 to 2 ratio reduces to 1 to 1.
  7. Benzene shares CH yet is a different molecule · One ratio can hide two different molecules.
  8. Multiply both numbers by 2 · Formulas need whole numbers, and doubling clears the half.
Worksheet · LightMySky