Molecular Mass and Formula Mass

General Chemistry • Chemical Compounds

Written by STEM Calculators Team Published October 15, 2025 Updated February 24, 2026

Build a chemical formula by choosing each element and its subscript. Then compute the relative molecular mass \(M_r\) (for molecules) or the relative formula mass \(M_r\) (for ionic compounds). Long math is stacked vertically.

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Accepts parentheses and Unicode subscripts (H₂O). Ionic charges/electrons are not supported.

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Duplicate elements are combined automatically.

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Relative molecular mass (M_r) and molar mass (M)

Relative molecular mass (M_r) and relative formula mass are found by adding the relative atomic masses of all atoms shown in a chemical formula. The calculator computes M_r (unitless) and the corresponding molar mass M (in g·mol⁻¹) using the same numeric value.

Core definition and essential equations

M_r is the total of each element’s relative atomic mass multiplied by how many times it appears in the formula.

\[ \begin{aligned} M_r &= \sum_i n_i\,A_{r,i} \\ M\ (\mathrm{g\cdot mol^{-1}}) &= M_r \end{aligned} \]

Here, \(A_{r,i}\) is the relative atomic mass of element \(i\) (from the periodic table), and \(n_i\) is the number of atoms of that element in the formula. For molecular substances, M_r is often called relative molecular mass; for ionic compounds, it is often called relative formula mass. Both use the same summation rule.

Interpreting the results

A larger M_r means heavier particles and a larger mass per mole. M_r has no units; molar mass \(M\) is expressed in \(\mathrm{g\cdot mol^{-1}}\) and represents the mass of \(1\ \mathrm{mol}\) of the substance. If the calculator shows a breakdown by elements, the largest contributions usually come from elements with higher \(A_r\) values or larger subscripts.

Common pitfalls

Forgetting to multiply by subscripts (e.g., O₂ means two O atoms).
Not distributing parentheses (e.g., (OH)₂ contains O₂ and H₂).
Ignoring hydrate water (e.g., ·5H₂O adds 10 H and 5 O atoms).
Mixing units: M_r is unitless, while \(M\) must be reported in \(\mathrm{g\cdot mol^{-1}}\).

Micro example

Carbon dioxide, CO₂:

\[ \begin{aligned} M_r(\mathrm{CO_2}) &= 1\cdot 12.011 + 2\cdot 15.999 \\ &= 44.009 \end{aligned} \]

Use this tool when a formula is known and the goal is to convert between mass and moles, compare particle masses, or prepare for stoichiometry and limiting-reagent work. It is not suitable for determining a formula from experimental data; a next-step concept in that case is empirical formula from percent composition (or isotopic/mass-spectrometry interpretation).

Frequently Asked Questions

What is the difference between molecular mass and formula mass?

Molecular mass applies to discrete molecules (like H2O), while formula mass applies to ionic compounds where the formula represents a formula unit (like CaCl2). Both are computed the same way: sum of atomic masses multiplied by subscripts.

How does this calculator compute M_r from a chemical formula?

It uses the weighted sum M_r = sum(n_i x A_i), where n_i is the subscript and A_i is the atomic mass of each element. The calculator adds each element contribution and reports the total.

Why does the molar mass have the same number as M_r?

Relative mass M_r is dimensionless, while molar mass M has units of g/mol. By definition, the numeric value of M in g/mol matches M_r for the same chemical formula.

What happens if I enter the same element twice?

Duplicate elements are combined automatically so the total count of that element is used in the sum. This helps prevent double-counting and keeps the formula consistent.

Which atomic masses does the calculator use?

It uses periodic-table atomic masses that are averaged over natural isotopic composition. These values are appropriate for typical molecular and formula mass calculations in general chemistry.

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