Index / Science
Molar Mass Calculator
A formula in, a molar mass out, with the percent composition and the arithmetic for a weighed sample. Brackets nest, hydrates are written with a dot, and every atomic weight it used is printed underneath.
Compound
Ca(OH)2, Fe2(SO4)3, KAl(SO4)2·12H2O and
2 NaHCO3 are all read. A dot, a full stop or an asterisk all
separate a hydrate.
Molar mass
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Atoms per formula unit
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Elements
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Composition
The sample
Which weights these are, and the three places they are not what people assume
These are the IUPAC standard atomic weights — the mean over
the isotopic composition of normal terrestrial material, not the mass of
any one atom. Chlorine is 35.45 because natural chlorine is a mixture;
no chlorine atom weighs that.
An element in square brackets has no stable isotope, so it has no standard atomic weight at all. What is shown for those is the mass number of the longest-lived isotope — technetium as 97, plutonium as 244 — which is a different kind of quantity wearing the same units. Any result containing one is marked.
Several elements genuinely vary by source. IUPAC publishes an interval rather than a number for lithium, boron, carbon, nitrogen, oxygen, silicon, sulfur, chlorine, bromine and thallium; the conventional single value is used here. Lithium is the extreme case: it runs from 6.938 to 6.997 depending on where the sample was mined, which is a 0.8% spread and larger than most of the arithmetic below.
The molar mass constant is no longer exactly 1 g/mol. Since the 2019 redefinition the mole is a fixed count — 6.02214076 × 1023, exactly — and carbon-12's molar mass is a measured 11.9999999958 g/mol rather than exactly 12. The difference is 3.5 parts in 1010 and matters to nobody weighing anything, but the definitions ran the other way round before 2019 and now they do not.
The molecule count is written out in full, every digit of it, because the Avogadro number is now an exact integer and there is no reason to round it into an exponent. It is computed on digit strings for the same reason: 6.02214076 × 1023 does not fit in a double without losing the last eight digits.
A charge is ignored, and saying so is the point. Sulfate written SO4^2- has two extra electrons, and an electron is 0.000548580 g/mol. For most work that is noise; for a mass spectrum it is not, and a calculator that silently drops it is hiding the one thing you asked about.
The formula is echoed back in Hill order — carbon first, then hydrogen, then everything else alphabetically — which is how chemical databases index a formula, and which is why sodium chloride comes back as ClNa. It is the same compound written the way an index would file it.
Symbols are case-sensitive, and this page will not guess. Co is cobalt and CO is carbon monoxide. A lower-case first letter is reported as an error rather than corrected, because correcting it is how you get the wrong compound with no warning.
An element in square brackets has no stable isotope, so it has no standard atomic weight at all. What is shown for those is the mass number of the longest-lived isotope — technetium as 97, plutonium as 244 — which is a different kind of quantity wearing the same units. Any result containing one is marked.
Several elements genuinely vary by source. IUPAC publishes an interval rather than a number for lithium, boron, carbon, nitrogen, oxygen, silicon, sulfur, chlorine, bromine and thallium; the conventional single value is used here. Lithium is the extreme case: it runs from 6.938 to 6.997 depending on where the sample was mined, which is a 0.8% spread and larger than most of the arithmetic below.
The molar mass constant is no longer exactly 1 g/mol. Since the 2019 redefinition the mole is a fixed count — 6.02214076 × 1023, exactly — and carbon-12's molar mass is a measured 11.9999999958 g/mol rather than exactly 12. The difference is 3.5 parts in 1010 and matters to nobody weighing anything, but the definitions ran the other way round before 2019 and now they do not.
The molecule count is written out in full, every digit of it, because the Avogadro number is now an exact integer and there is no reason to round it into an exponent. It is computed on digit strings for the same reason: 6.02214076 × 1023 does not fit in a double without losing the last eight digits.
A charge is ignored, and saying so is the point. Sulfate written SO4^2- has two extra electrons, and an electron is 0.000548580 g/mol. For most work that is noise; for a mass spectrum it is not, and a calculator that silently drops it is hiding the one thing you asked about.
The formula is echoed back in Hill order — carbon first, then hydrogen, then everything else alphabetically — which is how chemical databases index a formula, and which is why sodium chloride comes back as ClNa. It is the same compound written the way an index would file it.
Symbols are case-sensitive, and this page will not guess. Co is cobalt and CO is carbon monoxide. A lower-case first letter is reported as an error rather than corrected, because correcting it is how you get the wrong compound with no warning.