Which One of the Following Is Not an Empirical Formula?
An empirical formula shows the simplest whole-number ratio of atoms in a compound, so it cannot be reduced any further. When a question asks which one of the following is not an empirical formula, the answer is the formula whose subscripts share a common divisor.
What Is an Empirical Formula
An empirical formula gives the lowest whole-number ratio of the different atoms in a substance. It tells you what elements are present and their simplest proportion, but not the actual number of atoms in a molecule. For example, hydrogen peroxide has the empirical formula HO, even though its molecule contains two hydrogen and two oxygen atoms.
- Molecular formula — the actual number of each atom in one molecule, such as H₂O₂.
- Empirical formula — the reduced ratio only, such as HO.
Glucose makes the contrast clear. Its molecular formula is C₆H₁₂O₆, but all three subscripts divide evenly by 6, so its empirical formula is CH₂O. According to the Royal Society of Chemistry, the empirical formula is the simplest ratio and is frequently all you can determine from percentage composition data alone.
Identifying the Non-Empirical Formula
To test each option, follow these steps:
- List the subscripts for every element in the formula.
- Find the greatest common factor (GCF) of those numbers.
- If the GCF is 1, the formula is already in its simplest form and is empirical.
- If the GCF is greater than 1, divide every subscript by it — the original formula is not empirical.
Worked example. Which of these is not an empirical formula: CH₄, C₃H₆, NO₂, or H₂O?
- CH₄ → GCF of 1 and 4 is 1 → empirical.
- C₃H₆ → GCF of 3 and 6 is 3 → reduces to CH₂ → not empirical.
- NO₂ → GCF of 1 and 2 is 1 → empirical.
- H₂O → GCF of 2 and 1 is 1 → empirical.
So C₃H₆ is the answer. As Khan Academy notes, any formula whose subscripts can all be divided by the same number is a molecular formula rather than an empirical one.
Key Takeaways
- An empirical formula is the simplest whole-number ratio of atoms in a compound.
- A formula is not empirical if all its subscripts share a common factor greater than 1.
- Find the greatest common factor: if it is 1, the formula is empirical; if not, it can be reduced.
- Molecular and structural formulas show actual atom counts or bonding, while the empirical formula only shows the ratio.
