How to Calculate Molar Mass from a Chemical Formula

1. 1

   Write down the chemical formula correctly

   Enter the formula using standard chemical notation — uppercase for elements, lowercase where needed (e.g. H2O, NaHCO3, C6H12O6). Use parentheses for groups: Ca(OH)2, (NH4)2SO4. The tool handles nested parentheses and hydrates like CuSO4·5H2O using a dot separator.

2. 2

   Identify each element and its atom count

   Parse the formula to count how many atoms of each element are present. For H2O: 2 × H, 1 × O. For Ca(OH)2: 1 × Ca, 2 × O, 2 × H. The tool uses a deterministic parser that handles all common notation including polyatomic groups and coefficient multipliers.

3. 3

   Multiply by atomic weight and sum

   Look up each element's standard atomic weight from the IUPAC 2021 table — built into the tool and verified against NIST SRD 144. Multiply each count by the weight and add them up: H2O = 2×1.008 + 1×15.999 = 18.015 g/mol. The tool displays the per-element breakdown for each line of your result.

FAQ

What is the difference between molar mass and molecular weight?

Molecular weight is the mass of a single molecule in atomic mass units (amu or Da). Molar mass is the mass of one mole (6.022×10²³ molecules) in grams per mole (g/mol). Numerically they are identical — the difference is only the unit. In practice, chemists use 'molar mass' for lab-scale work and 'molecular weight' interchangeably.

How do hydrates and dot formulas like CuSO4·5H2O work?

Separate the anhydrous compound and the water with a dot (·) or asterisk (*). The tool adds the mass of the water molecules to the main compound: CuSO4·5H2O = CuSO4 + 5 × H2O. Both the total and the anhydrous-base molar mass are shown separately.

Which atomic weights are used — and how often are they updated?

The tool uses IUPAC 2021 standard atomic weights (CIAAW), verified against NIST Atomic Weights and Isotopic Compositions database. Atomic weights are periodically revised by IUPAC — the last major update was in 2021. The methodology version and last-verified date are displayed on the tool page for full transparency.

Can I calculate molar mass for charged species or isotopes?

The current version handles neutral compounds and common ionic formulas (e.g. Na+, SO4²⁻). Isotope-specific masses are not yet supported — the tool uses the terrestrial average atomic weight for each element. For isotopic calculations, use the NIST Isotopic Mass Calculator.