Chapter 6 · Chemical Composition
6.8Calculating Empirical Formulas for Compounds
14 min · two checks
Predict
A compound is 40.0% C, 6.7% H, and 53.3% O. What is the empirical formula?
The idea
- Turn mass percent or mass data into an empirical formula.
- Multiply a ratio like 1 : 1.5 up to whole numbers.
Percent data become masses by assuming a 100 g sample: 40.0% C is 40.0 g C. Convert each mass to moles with that element’s molar mass. Divide every mole amount by the smallest one. If the results are all integers, within experimental scatter such as 1.98 or 3.02, those integers are the subscripts. If a result is near 1.5, 1.33, or 1.25, multiply all subscripts by 2, 3, or 4 to clear the fraction.
The empirical formula is the simplest ratio, not automatically the molecular formula. CH₂O is the empirical formula of formaldehyde, acetic acid (C₂H₄O₂), and glucose (C₆H₁₂O₆). Composition alone cannot tell those apart. Also do not use the periodic-table atomic number as the mass. Carbon is 12.01 g/mol, not 6.
Keep these
- Assume 100 g when you are given percents.
- Moles, then divide by the smallest, then clear fractions.
- Empirical formulas hide the true multiple.
Worked path
A sample is 25.9 g nitrogen and 74.1 g oxygen. Find the empirical formula. N 14.01, O 16.00.
Observe
Masses are already in grams, so do not assume a new 100 g.