empirical formula
Summary
The empirical formula gives the simplest whole-number ratio of atoms of each element in a compound. A molecular formula is a whole-number multiple of it, so different compounds can share one (C₂H₂ and C₆H₆ are both CH). Finding it from mass data is the usual task.
Detail
Meaning: The empirical formula shows the smallest whole-number ratio of elements in a compound. To find it, treat percent composition as grams in a 100 g sample, convert each mass to moles by dividing by molar mass, then divide every mole value by the smallest one. If a ratio ends near .5, .33 or .25, multiply all values by 2, 3 or 4 to reach whole numbers. For ionic compounds, the formula unit (NaCl, CaCl₂) is already the empirical formula.
Equation: molecular formula = (empirical formula)n, where n = molar mass of the compound (g/mol) ÷ empirical formula mass (g/mol). n is a dimensionless whole number.
On the MCAT: Passages give percent composition or combustion analysis data. In combustion, the carbon in CO₂ and the hydrogen in H₂O are found from their masses, and oxygen is often found by difference. A separate molar mass, such as from mass spectrometry, is then used to get n and the molecular formula.
Common confusion: Students round 1.5 to 2 or 1.33 to 1, which gives the wrong ratio. Only values very close to a whole number (about 0.1 or less away) should be rounded. Another mistake is treating the empirical formula as the actual composition of a molecule. Glucose (C₆H₁₂O₆) and formaldehyde (CH₂O) share an empirical formula, so the molar mass is needed to settle the molecular formula.
Sources
- Chemistry: The Central Science
- Chemistry: Atoms First
- General Chemistry: Principles and Modern Applications
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.