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Molecular weight calculator

Type any chemical formula to get its molar mass in g/mol, the mass percent of each element and a quick grams to moles conversion. Brackets, hydrates such as CuSO4路5H2O and complex ions such as K4[Fe(CN)6] are all understood.

Case matters (Co is cobalt, CO is carbon monoxide). Use brackets for groups, a dot or * for hydrates and ^ for charges, such as SO4^2-.

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Molar mass

249.677g/mol

Hill formula CuH10O9S21 atoms
Water of crystallisation: 5 H鈧侽 = 90.075 g/mol, or 36.08 % of the mass. This is the theoretical water content you would expect from a Karl Fischer or loss on drying test.

Grams and moles

g
mol

n = m 梅 M. 10 g 梅 249.677 g/mol = 0.040052 mol (40.052 mmol).

Compounds with formula CuH10O9S Find suppliers

Composition by mass

ElementAtomsMassMass %
O9143.99157.67
Cu163.54625.45
S132.06012.84
H1010.0804.04
Total21249.677100.00

Read as CuSO4 (159.602 g/mol) + 5 H2O (90.075 g/mol).

How the calculation works

The molar mass of a compound is the sum of the atomic weights of all the atoms in its formula. For each element, multiply its standard atomic weight by the number of atoms of that element, then add the results.

M = 危 ( ni 脳 Ar,i )
Molar mass in g/mol, summed over every element i in the formula
M
molar mass of the compound, in g/mol
ni
number of atoms of element i in one formula unit
Ar,i
standard atomic weight of element i (dimensionless, numerically equal to g/mol)

The calculator reads the formula from left to right. A number after an element or a closing bracket multiplies it, so in Ca3(PO4)2 the phosphate group counts twice: 2 phosphorus and 8 oxygen atoms. Brackets can be nested with ( ), [ ] or { }. A dot, a middle dot or an asterisk starts a new part, and a number at the start of a part multiplies the whole part, which is how hydrates and adducts such as Na2CO3路10H2O are written.

Mass percent tells you how much of the total mass comes from each element: mass % = n 脳 A 梅 M 脳 100. It is the basis of elemental analysis results and of the theoretical metal or water content quoted on many certificates of analysis.

Atomic weights follow the abridged standard atomic weights published by IUPAC (CIAAW). Elements such as hydrogen, carbon and chlorine vary slightly in nature, so IUPAC gives an interval and a conventional value for everyday use, and the calculator uses those conventional values.

Atomic weights of all 118 elementsValues used by this calculator. Bracketed values are mass numbers of long lived isotopes.
  • 1HHydrogen1.008
  • 2HeHelium4.0026
  • 3LiLithium6.94
  • 4BeBeryllium9.0122
  • 5BBoron10.81
  • 6CCarbon12.011
  • 7NNitrogen14.007
  • 8OOxygen15.999
  • 9FFluorine18.998
  • 10NeNeon20.18
  • 11NaSodium22.99
  • 12MgMagnesium24.305
  • 13AlAluminium26.982
  • 14SiSilicon28.085
  • 15PPhosphorus30.974
  • 16SSulfur32.06
  • 17ClChlorine35.45
  • 18ArArgon39.95
  • 19KPotassium39.098
  • 20CaCalcium40.078
  • 21ScScandium44.956
  • 22TiTitanium47.867
  • 23VVanadium50.942
  • 24CrChromium51.996
  • 25MnManganese54.938
  • 26FeIron55.845
  • 27CoCobalt58.933
  • 28NiNickel58.693
  • 29CuCopper63.546
  • 30ZnZinc65.38
  • 31GaGallium69.723
  • 32GeGermanium72.63
  • 33AsArsenic74.922
  • 34SeSelenium78.971
  • 35BrBromine79.904
  • 36KrKrypton83.798
  • 37RbRubidium85.468
  • 38SrStrontium87.62
  • 39YYttrium88.906
  • 40ZrZirconium91.224
  • 41NbNiobium92.906
  • 42MoMolybdenum95.95
  • 43TcTechnetium[98]
  • 44RuRuthenium101.07
  • 45RhRhodium102.91
  • 46PdPalladium106.42
  • 47AgSilver107.87
  • 48CdCadmium112.41
  • 49InIndium114.82
  • 50SnTin118.71
  • 51SbAntimony121.76
  • 52TeTellurium127.6
  • 53IIodine126.9
  • 54XeXenon131.29
  • 55CsCaesium132.91
  • 56BaBarium137.33
  • 57LaLanthanum138.91
  • 58CeCerium140.12
  • 59PrPraseodymium140.91
  • 60NdNeodymium144.24
  • 61PmPromethium[145]
  • 62SmSamarium150.36
  • 63EuEuropium151.96
  • 64GdGadolinium157.25
  • 65TbTerbium158.93
  • 66DyDysprosium162.5
  • 67HoHolmium164.93
  • 68ErErbium167.26
  • 69TmThulium168.93
  • 70YbYtterbium173.05
  • 71LuLutetium174.97
  • 72HfHafnium178.49
  • 73TaTantalum180.95
  • 74WTungsten183.84
  • 75ReRhenium186.21
  • 76OsOsmium190.23
  • 77IrIridium192.22
  • 78PtPlatinum195.08
  • 79AuGold196.97
  • 80HgMercury200.59
  • 81TlThallium204.38
  • 82PbLead207.2
  • 83BiBismuth208.98
  • 84PoPolonium[209]
  • 85AtAstatine[210]
  • 86RnRadon[222]
  • 87FrFrancium[223]
  • 88RaRadium[226]
  • 89AcActinium[227]
  • 90ThThorium232.04
  • 91PaProtactinium231.04
  • 92UUranium238.03
  • 93NpNeptunium[237]
  • 94PuPlutonium[244]
  • 95AmAmericium[243]
  • 96CmCurium[247]
  • 97BkBerkelium[247]
  • 98CfCalifornium[251]
  • 99EsEinsteinium[252]
  • 100FmFermium[257]
  • 101MdMendelevium[258]
  • 102NoNobelium[259]
  • 103LrLawrencium[266]
  • 104RfRutherfordium[267]
  • 105DbDubnium[268]
  • 106SgSeaborgium[269]
  • 107BhBohrium[270]
  • 108HsHassium[269]
  • 109MtMeitnerium[277]
  • 110DsDarmstadtium[282]
  • 111RgRoentgenium[282]
  • 112CnCopernicium[286]
  • 113NhNihonium[286]
  • 114FlFlerovium[290]
  • 115McMoscovium[290]
  • 116LvLivermorium[293]
  • 117TsTennessine[294]
  • 118OgOganesson[295]

Worked example

The same steps the calculator follows, done by hand

Copper(II) sulfate pentahydrate, the blue crystals sold for agriculture and electroplating, has the formula CuSO4路5H2O. Count the atoms first: 1 Cu, 1 S, 4 + 5 = 9 O and 5 脳 2 = 10 H.

ElementAtomsAtomic weightMass (g/mol)
Copper (Cu)163.54663.546
Sulfur (S)132.0632.060
Oxygen (O)915.999143.991
Hydrogen (H)101.00810.080
Total21249.677
  1. 1
    Add the element masses: 63.546 + 32.060 + 143.991 + 10.080 = 249.677 g/mol.
  2. 2
    The five waters weigh 5 脳 18.015 = 90.075 g/mol, so the crystals contain 90.075 梅 249.677 脳 100 = 36.1 % water by mass.
  3. 3
    To make a solution containing 0.1 mol of copper sulfate you would weigh 0.1 脳 249.677 = 24.97 g of the pentahydrate, not 15.96 g as you would for the anhydrous salt (159.60 g/mol).

Frequently asked questions

Is molecular weight the same as molar mass?
In everyday use, yes. Molecular weight (relative molecular mass) is a dimensionless ratio to one twelfth of a carbon-12 atom, and molar mass is the mass of one mole in g/mol. The two numbers are the same, so 18.015 for water means 18.015 g/mol.
How do I enter a hydrate or a salt with water of crystallisation?
Write the salt, then a dot, a middle dot (路) or an asterisk, then the number of water molecules, for example CuSO4路5H2O, CuSO4.5H2O or CuSO4*5H2O. The calculator also shows the theoretical water content in percent.
Why does my result differ slightly from a supplier label or a database?
Different sources round atomic weights differently or use older IUPAC values, so the second or third decimal place can differ. For weighing and quoting, two decimals are almost always enough. Larger gaps usually mean a different form, such as anhydrous versus hydrate or a salt versus the free acid.
Can I calculate the mass of an ion?
Yes. Add the charge at the end, such as SO4^2-, NH4+ or [Fe(CN)6]4-. The charge is shown but does not change the result, because the mass of an electron (about 0.00055 g/mol) is negligible.
What is the Hill formula shown under the result?
The Hill system writes carbon first, then hydrogen, then the other elements in alphabetical order. Compounds without carbon are written fully alphabetically. Chemical databases and catalogues use it, so it is the best form for searching.

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