formula

Meaning: A chemical formula lists the elements in a compound, with subscripts giving how many atoms of each. The molecular formula gives the actual atoms per molecule (glucose, C₆H₁₂O₆). The empirical formula gives the smallest whole-number ratio (CH₂O). Ionic compounds such as NaCl or CaCl₂ have no discrete molecules, so their formula is the lowest ratio of ions and is called a formula unit. Formula mass is the sum of the atomic masses in the formula.

Equation: molecular formula = (empirical formula)ₙ, where n = molar mass of the compound ÷ empirical formula mass. Molar masses are in g/mol (SI: kg/mol), and n is a unitless whole number.

On the MCAT: A passage gives percent composition or combustion-analysis data. Assume a 100 g sample, convert each mass to moles, divide all by the smallest value, and multiply by a small integer to clear fractions (1.5 → ×2, 1.33 → ×3). A given molar mass then lets you find n and the molecular formula. Formula mass is also used in stoichiometry and in converting grams to moles.

Common confusion: The empirical formula is not always the real molecule. Glucose and formaldehyde both reduce to CH₂O, so percent composition alone cannot tell them apart; the molar mass settles it. Also, do not round a ratio like 1.5 to 2, because that gives the wrong formula.