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Elemental Composition of Pure Substances

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A pure substance has a fixed composition, and that composition can be written three ways: a percent composition by mass, an empirical formula giving the smallest whole-number ratio of atoms, and a molecular formula giving the actual count per molecule. Moving between them runs through moles, because a formula counts atoms while a percentage weighs them.

UNIT 1 TOPIC 1.3 • ELEMENTAL COMPOSITION OF PURE SUBSTANCES FORMULA DETECTIVE Find the empirical formula of a pure compound from its percent composition. ELEMENT STEP 1 MASS % given STEP 2 IN 100 g grams = % STEP 3 AMOUNT (mol) ÷ molar mass STEP 4 ÷ SMALLEST ÷ 3.33 mol STEP 5 RATIO whole # C Carbon 40.0% 40.0 g 40.0 ÷ 12.01 = 3.33 mol 3.33 ÷ 3.33 = 1.00 1 H Hydrogen 6.7% 6.7 g 6.7 ÷ 1.008 = 6.65 mol 6.65 ÷ 3.33 = 2.00 2 O Oxygen 53.3% 53.3 g 53.3 ÷ 16.00 = 3.33 mol 3.33 ÷ 3.33 = 1.00 1 Σ % = 100.0% EMPIRICAL FORMULA CH₂O C : H : O = 1 : 2 : 1 EMPIRICAL vs MOLECULAR Empirical = simplest whole-number ratio (from % data). Molecular = a whole-number multiple of that unit. glucose C₆H₁₂O₆ = (CH₂O)₆ molar masses (g/mol): C 12.01 · H 1.008 · O 16.00 CED ANCHOR Law of definite proportions A pure compound always has the same elements in the same mass ratio, so the same percent composition always gives the same empirical formula — regardless of sample size or source. AP Chemistry · Unit 1 · Atomic Structure & Properties
The empirical-formula pipeline: assume a 100 g sample so each mass percent becomes grams, divide by molar mass to get moles of each element, then scale the mole amounts to the smallest whole-number ratio. That ratio, not the masses, is the formula.
Formula Detective · Open the sandbox →

The traps here all live at the boundary between mass and count: reading a mass percent as if it were a headcount of atoms, rounding a mole ratio before it has settled, and stopping at an empirical formula when the molecular one was asked for. The ratio of moles, reduced to whole numbers, is what a formula records.

The work

3 ways in · any order
Lesson
Elemental Composition ›

Percent composition, empirical formula, and molecular formula are three views of one fixed composition. The lesson moves between them through the mole, then closes with a ten-scenario applet that builds a formula from mass data one step at a time.

Skill check · 10 scenarios
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Diagnostic
10-item topic check ›

Ten items spanning the four Topic 1.3 misconceptions: mass percent read as an atom count, a mole ratio rounded too early, an empirical formula mistaken for the molecular one, and a non-simplest ratio left unreduced.

Not started · 10 items · ~15 min
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Targeted Practice
Drill a single misconception ›

Pick one of the failure modes you missed and drill it on its own. The round is adaptive: two correct in a row clears the misconception and moves you to the next.

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