2.3.5—Empirical and molecular formulas

To calculate an empirical formula from composition data, first record the mass or percentage of each element. Convert each value to amount using its relative atomic mass, divide every amount by the smallest amount, and scale the ratios to the smallest whole numbers that fit the data.

Keep the element order and units consistent, then check that the final integer ratio reproduces the supplied composition within the stated precision. Do not round each raw ratio independently before comparing the ratios; use a common multiplier when a simple fraction is present.

If a relative molecular mass is supplied, calculate the empirical-formula mass and find the whole-number multiple: multiplier = relative molecular mass ÷ empirical-formula mass. Multiply every empirical subscript by that same integer to obtain the molecular formula.

An empirical formula is the simplest ratio, whereas a molecular formula gives the actual atom numbers in one molecule. Do not use the molecular-multiple step for an ionic lattice, alter individual subscripts, or extend the calculation into reacting-mass or percentage-yield problems.