Empirical Formula Composition

\[r=\frac{m_i\,MW_{ref}}{m_{ref}\,MW_i}\]

Variables

rrelative mole ratio

mimass of element i (g)

mrefmass of reference element (g)

MWimolar mass of element i (g/mol)

MWrefmolar mass of reference element (g/mol)

Description

What is this formula?

This formula calculates the relative mole ratio between an element and a reference element when determining an empirical formula.

When to use it

Use this equation after converting elemental composition data into masses. The resulting mole ratios are used to determine the simplest whole-number ratio of atoms in a compound.

Example

Given:

Carbon mass = 40.0 g

Hydrogen mass = 6.7 g

MW(C) = 12.011 g/mol

MW(H) = 1.008 g/mol

Using hydrogen as reference:

r = (40.0×1.008)/(6.7×12.011)

Result:

r ≈ 0.50

This indicates a 1:2 ratio after normalization, consistent with the empirical formula CH2.

Applications

Empirical formula determination

Elemental analysis

Analytical chemistry

Stoichiometric calculations

Chemical education

Note

Determining an empirical formula is fundamentally a multi-step procedure rather than a single equation. This formula represents the core mole-ratio calculation used during the process. After obtaining all ratios, the values are normalized and converted to the smallest whole-number atomic ratio.