Molar Mass Converter — formula to g/mol, grams to moles

The molar mass converter above parses a chemical formula, sums the standard atomic weights of every atom in it, and uses the result to convert between mass, amount of substance and number of entities. Type a formula and the tool handles the rest, including nested parentheses and hydrate dots, so you are not counting atoms by hand or looking up a table entry that may not exist for your compound.

Arb Digital publishes this converter as part of a free technical reference library. The atomic weights are the conventional values recommended by the IUPAC Commission on Isotopic Abundances and Atomic Weights, and the Avogadro constant is the exact defined value fixed in the 2019 revision of the SI.

What This Molar Mass Converter Does

Enter a formula and the parser walks it left to right, reading element symbols as a capital letter optionally followed by a lowercase letter, applying subscripts, and recursing into any parenthesised or bracketed group before multiplying by that group's own subscript. A dot separates hydrate components, and a leading integer on a component multiplies the whole of it, so CuSO4.5H2O gives 249.7 g/mol rather than the anhydrous 159.6.

The molar mass then drives the unit conversion. Mass units, from micrograms up to pounds, and amount units, from picomoles up to kilomoles, appear in a single list, along with a count of entities. Because the molar mass connects them, you can convert 180 grams of glucose to 0.999 moles or 6.02 × 10²³ molecules in one step. The four supporting panels always show the molar mass, the amount in moles, the mass in grams and the number of entities, whichever direction you converted in.

How to Use It

1. Type the formula exactly as written in chemistry, respecting capitalisation. Co is cobalt; CO is carbon monoxide, and the two differ by more than a factor of two in molar mass.
2. Check the molar mass panel before going further. If the formula contains an unrecognised symbol the tool says so rather than returning a plausible wrong number.
3. Enter the amount you have in grams, moles, or any of the other supported units.
4. Choose the target unit. Converting a mass into moles, or moles into a count of molecules, uses the molar mass and the Avogadro constant automatically.
5. Read the four panels for molar mass, moles, grams and entity count together, which covers almost every question a stoichiometry problem asks.

The Formula and How It Is Calculated

Molar mass is the sum of the standard atomic weights of all constituent atoms, expressed in grams per mole. For glucose, C₆H₁₂O₆, that is six carbons at 12.011, twelve hydrogens at 1.008 and six oxygens at 15.999, giving 180.156 g/mol.

From there, two relationships do all the work. Amount of substance is mass divided by molar mass, n = m ÷ M, and the number of entities is that amount multiplied by the Avogadro constant, N = n × N_A. Since the 2019 revision of the SI, the mole is defined by fixing N_A at exactly 6.02214076 × 10²³ per mole, as set out in the BIPM SI Brochure. The atomic weight values are those published by CIAAW, the IUPAC commission responsible for them, and unit style follows NIST Special Publication 811.

Standard Atomic Weights Are Averages, Not Constants

An atomic weight in a periodic table is not a property of an atom. It is a weighted average over the isotopes found in normal terrestrial material. Chlorine is listed as 35.45 because natural chlorine is roughly three parts chlorine-35 to one part chlorine-37; no individual chlorine atom has that mass.

For several elements the natural abundance varies measurably between sources, and IUPAC now publishes those atomic weights as intervals rather than single numbers. Hydrogen, lithium, boron, carbon, nitrogen, oxygen, silicon, sulfur, chlorine, bromine and thallium all fall into this category. Carbon, for example, ranges from about 12.0096 to 12.0116 depending on whether the sample is biological, geological or industrial. This converter uses the conventional single values IUPAC provides for exactly this purpose, which are appropriate for ordinary laboratory and teaching work, but they carry an implicit uncertainty in the fourth or fifth significant figure. Anyone doing isotope-ratio work needs measured values for their specific material, not table values.

Molar Mass, Molecular Weight and Formula Mass

Three terms circulate for closely related ideas and they are not perfectly interchangeable. Molar mass is a physical quantity with units of grams per mole and applies to any chemical entity. Molecular weight, more properly relative molecular mass, is a dimensionless ratio against one twelfth of the mass of a carbon-12 atom, so it has the same numerical value but no units.

Formula mass is the term to use for substances that do not contain discrete molecules. Sodium chloride is an ionic lattice; there is no NaCl molecule, only a repeating three-dimensional arrangement in a one-to-one ratio. Its formula mass of 58.44 describes the mass of one mole of formula units, which is the useful quantity even though the molecule it names does not exist. Polymers present the opposite issue: they have a distribution of chain lengths, so a single molar mass has to be qualified as a number average or a weight average. This tool computes the formula-based value, which is exact for a defined composition and only an average for a polymer.

What the 2019 Redefinition Changed

Before 2019 the mole was defined as the number of atoms in exactly 12 grams of carbon-12. That made the molar mass of carbon-12 exactly 12 g/mol by definition and left the Avogadro constant as a measured quantity. The revision inverted the arrangement: the Avogadro constant is now exactly 6.02214076 × 10²³ per mole, and the molar mass of carbon-12 became a measured quantity.

The practical effect is almost nil, but the conceptual effect is worth understanding. The molar mass constant, the factor connecting the dimensionless relative atomic mass to grams per mole, is no longer exactly 1 g/mol. It is 1 g/mol to within about three parts in ten billion, far below the uncertainty of any atomic weight in the table. So the arithmetic every chemistry course teaches remains correct to any precision you will ever use, but "molar mass in g/mol equals atomic weight" is now an excellent approximation rather than a definition.

Average Mass Versus Monoisotopic Mass

Mass spectrometry exposes a distinction that ordinary stoichiometry hides. The molar mass this tool computes is an average over natural isotopic abundance. A mass spectrometer separates individual ions, so it sees discrete peaks corresponding to specific isotopic compositions, and the peak of interest is usually the monoisotopic mass, calculated using only the lightest common isotope of each element.

For glucose the average molar mass is 180.156 while the monoisotopic mass is about 180.063, a difference of roughly 0.05%. That is negligible in a weighing but enormous at the resolving power of a modern instrument, where it decides which formula a measured mass corresponds to. The gap widens with molecular size and with elements that have heavy minor isotopes, such as chlorine or bromine, whose isotope patterns produce a characteristic cluster of peaks rather than a single line. Use average molar mass for weighing out reagents and making solutions; use monoisotopic mass for interpreting spectra. For preparing those solutions, our concentration converter takes the molar mass calculated here and turns it into molarity.

Molar Masses of Common Compounds

Units supported by the converter, with the quantity each belongs to:

Common Mistakes to Avoid

• Getting the capitalisation wrong — CO is carbon monoxide at 28.01 g/mol while Co is cobalt at 58.93, and a parser cannot tell which you meant.
• Forgetting the water of crystallisation — copper sulfate pentahydrate is 249.7 g/mol against 159.6 for the anhydrous salt, a 56% weighing error.
• Using average molar mass to interpret a mass spectrum — spectra need monoisotopic masses, which differ by about 0.05% for organic molecules.
• Calling an ionic compound a molecule — sodium chloride has a formula mass, not a molecular mass, because no discrete NaCl molecule exists.
• Quoting a single molar mass for a polymer — chain length distributions require a number average or weight average, not one value.

Related Free Tools From Arb Digital

Molar mass feeds directly into several other calculations. Turn it into solution strength with the concentration converter, weigh reagents with the weight converter, measure solvent volumes with the volume converter, and handle solution density in the density converter. Molar heat capacity is covered by the specific heat converter, huge entity counts are easier to read through the scientific notation converter, and the general unit converter covers the rest.

Frequently Asked Questions

This tool is a calculation aid for unit conversion. Reagent preparation, dosing, and any safety-critical chemistry should be checked against the supplier certificate of analysis and the relevant safety data sheet.