Chemistry topic

Relative Formula Mass

Relative formula mass (Mr) is the sum of the relative atomic masses of all atoms in a formula. Calculate it, then convert between mass, moles, and particles.

Calculators

Molecular Weight CalculatorEnter a chemical formula. The calculator gives its molar mass, molecular weight, atom count, and element composition.Molar Mass CalculatorCalculate molar mass from an explicit chemical formula using versioned atomic-weight data.Grams to Moles CalculatorConvert a sample mass to moles from its chemical formula or molar mass.Moles to Grams CalculatorConvert moles to a selected mass unit from a chemical formula or molar mass.Moles CalculatorCalculate moles from an amount, sample mass, or particle count.Percent Composition and Empirical FormulaFind the empirical formula from percent composition or element masses. See the percent composition formula and worked problems.Find Molecular Formula from Empirical FormulaFind the molecular formula from the empirical formula and molar mass. Divide the molar mass by the empirical formula mass, then multiply the subscripts.

What is relative formula mass?

Relative formula mass, Mr, is the sum of the relative atomic masses, Ar, of all the atoms in a chemical formula.

Mr has no unit. It compares the mass of one formula unit with one-twelfth of the mass of a carbon-12 atom.

The same number in grams per mole is the molar mass. Water has Mr = 18, so its molar mass is 18 g/mol.

Chemists say relative formula mass for ionic compounds such as NaCl and relative molecular mass for molecules. The calculation is the same.

How to calculate relative formula mass

Mr = Σ (number of atoms × Ar)

Write the chemical formula and count the atoms of each element.
Multiply out brackets: Ca(OH)2 contains 1 Ca, 2 O, and 2 H.
Find the relative atomic mass, Ar, of each element on the periodic table.
Multiply each Ar by its number of atoms, then add the results.

Relative formula mass examples

These examples use the rounded Ar values on most exam data sheets: H = 1, C = 12, N = 14, O = 16, Na = 23, Mg = 24, S = 32, Cl = 35.5, Ca = 40.

Substance	Formula	Calculation	Mr
Water	H2O	2 × 1 + 16	18
Carbon dioxide	CO2	12 + 2 × 16	44
Sodium chloride	NaCl	23 + 35.5	58.5
Magnesium oxide	MgO	24 + 16	40
Calcium carbonate	CaCO3	40 + 12 + 3 × 16	100
Sulfuric acid	H2SO4	2 × 1 + 32 + 4 × 16	98
Calcium hydroxide	Ca(OH)2	40 + 2 × (16 + 1)	74
Ammonium nitrate	NH4NO3	2 × 14 + 4 × 1 + 3 × 16	80

Percentage by mass from relative formula mass

Percentage by mass of an element = (Ar × number of atoms ÷ Mr) × 100

Carbon in CO2: 12 ÷ 44 × 100 = 27.3%. Oxygen in CO2: 2 × 16 ÷ 44 × 100 = 72.7%.

Relative formula mass, formula mass, and molar mass

These terms share one number. They differ only in their units and in what they describe.

Term	Symbol	Unit	Water
Relative formula mass	Mr	No unit	18.02
Formula mass	None	u (atomic mass unit)	18.02 u
Molecular weight	MW	g/mol or Da	18.02 g/mol
Molar mass	M	g/mol	18.02 g/mol

Core relationships

These equations form an accessible map of the quantities used across this topic.

Mr = Σ(nᵢ × Arᵢ)
M = Mr g/mol
n = m/M
N = nNₐ

Adjacent topics

Reactions & Stoichiometry Analytical Chemistry