Which of the following CANNOT be an empirical formula?

Analyze the question: An empirical formula shows the simplest whole-number ratio of atoms in a compound. So the formula that CANNOT be empirical is one whose subscripts can be reduced further.

Concept: Divide all the subscripts by their highest common factor. If the ratio can be reduced, the formula is not in its simplest form, so it is not empirical.

Check each option:

• A. CH: ratio 1:1, already the simplest. It can be empirical (e.g. the empirical formula of benzene, C₆H₆).
• B. CH₂: ratio 1:2, already the simplest. It is the empirical formula of alkenes.
• C. P₂O₅: ratio 2:5, and 2 and 5 share no common factor, so it is already the simplest. (The molecular formula is P₄O₁₀, but P₂O₅ is the empirical formula.)
• D. N₂O₄: ratio 2:4, and both are divisible by 2, so it reduces to NO₂. N₂O₄ is therefore a molecular formula, not an empirical one.

Answer: The formula that cannot be an empirical formula is N₂O₄, because it can be simplified to NO₂.

Tip: In "which cannot be empirical" questions, look for subscripts that share a common factor (like 2 and 4). If they do, the formula can still be reduced, so it is not empirical.