Molar Mass Calculator
Molar mass of a chemical formula in g/mol from IUPAC atomic weights, with each element’s mass percent. Reads brackets and hydrates such as CuSO4·5H2O. How it works
Use the Molar Mass Calculator
About the Molar Mass Calculator
Water, H2O, has a molar mass of 18.015 g/mol: 2 × 1.008 for hydrogen plus 15.999 for oxygen. This molar mass calculator adds the IUPAC atomic weights of every atom in a formula, reads brackets and hydrate dots, and lists each element’s share of the mass. For a compound the molar mass in g/mol is the same number as its molecular weight or formula weight in atomic mass units.
Molar masses of common compounds
| Compound | Formula | Molar mass (g/mol) | Mass percent by element |
|---|---|---|---|
| Water | H2O | 18.015 | H 11.19%, O 88.81% |
| Carbon dioxide | CO2 | 44.009 | C 27.29%, O 72.71% |
| Carbon monoxide | CO | 28.010 | C 42.88%, O 57.12% |
| Oxygen | O2 | 31.998 | O 100.00% |
| Nitrogen | N2 | 28.014 | N 100.00% |
| Methane | CH4 | 16.043 | C 74.87%, H 25.13% |
| Ammonia | NH3 | 17.031 | N 82.24%, H 17.76% |
| Sodium chloride | NaCl | 58.44 | Na 39.34%, Cl 60.66% |
| Hydrochloric acid (hydrogen chloride) | HCl | 36.46 | H 2.76%, Cl 97.24% |
| Sodium hydroxide | NaOH | 39.997 | Na 57.48%, O 40.00%, H 2.52% |
| Sodium bicarbonate | NaHCO3 | 84.006 | Na 27.37%, H 1.20%, C 14.30%, O 57.14% |
| Calcium carbonate | CaCO3 | 100.086 | Ca 40.04%, C 12.00%, O 47.96% |
| Calcium hydroxide | Ca(OH)2 | 74.092 | Ca 54.09%, O 43.19%, H 2.72% |
| Sulfuric acid | H2SO4 | 98.07 | H 2.06%, S 32.69%, O 65.25% |
| Ethanol | C2H5OH | 46.069 | C 52.14%, H 13.13%, O 34.73% |
| Acetic acid | CH3COOH | 60.052 | C 40.00%, H 6.71%, O 53.28% |
| Glucose | C6H12O6 | 180.156 | C 40.00%, H 6.71%, O 53.28% |
| Sucrose | C12H22O11 | 342.297 | C 42.11%, H 6.48%, O 51.41% |
| Aspirin | C9H8O4 | 180.159 | C 60.00%, H 4.48%, O 35.52% |
| Iron(III) oxide | Fe2O3 | 159.687 | Fe 69.94%, O 30.06% |
| Potassium permanganate | KMnO4 | 158.032 | K 24.74%, Mn 34.76%, O 40.50% |
| Ammonium sulfate | (NH4)2SO4 | 132.13 | N 21.20%, H 6.10%, S 24.26%, O 48.43% |
| Iron(III) sulfate | Fe2(SO4)3 | 399.86 | Fe 27.93%, S 24.05%, O 48.01% |
| Copper(II) sulfate, anhydrous | CuSO4 | 159.60 | Cu 39.82%, S 20.09%, O 40.10% |
| Copper(II) sulfate pentahydrate | CuSO4·5H2O | 249.68 | Cu 25.45%, S 12.84%, O 57.67%, H 4.04% |
| Potassium ferrocyanide, anhydrous | K4[Fe(CN)6] | 368.346 | K 42.46%, Fe 15.16%, C 19.56%, N 22.82% |
Each total has as many decimals as the atomic weight with the fewest decimals in the formula, so anything with sulfur (32.06) or chlorine (35.45) shows two. Glucose and acetic acid share the same percentages because both have the ratio CH2O.
Atomic weights the calculator uses
| Element | Symbol | Atomic weight shown | How CIAAW lists it |
|---|---|---|---|
| Hydrogen | H | 1.008 | Interval; abridged value used |
| Carbon | C | 12.011 | Interval; abridged value used |
| Nitrogen | N | 14.007 | Interval; abridged value used |
| Oxygen | O | 15.999 | Interval; abridged value used |
| Sodium | Na | 22.9898 | Single value |
| Magnesium | Mg | 24.305 | Interval; abridged value used |
| Phosphorus | P | 30.9738 | Single value |
| Sulfur | S | 32.06 | Interval; abridged value used |
| Chlorine | Cl | 35.45 | Interval; abridged value used |
| Potassium | K | 39.0983 | Single value |
| Argon | Ar | 39.95 | Interval; abridged value used |
| Calcium | Ca | 40.078 | Single value |
| Iron | Fe | 55.845 | Single value |
| Copper | Cu | 63.546 | Single value |
| Zinc | Zn | 65.38 | Single value |
| Selenium | Se | 78.971 | Single value |
| Silver | Ag | 107.8682 | Single value |
| Iodine | I | 126.9045 | Single value |
| Gold | Au | 196.9666 | Single value |
| Lead | Pb | 207.2 | Interval; abridged value used |
| Uranium | U | 238.0289 | Single value |
Single values are the CIAAW 2024 standard atomic weights to the digit of their stated uncertainty, shown to at most 4 decimals (sodium is stored as 22.98976928). For elements whose isotope mix varies in nature CIAAW gives an interval, for example chlorine [35.446, 35.457]; the calculator uses the CIAAW abridged value, so a real sample can differ in the last digit shown.
Frequently asked questions
- What is the molar mass of CO2?
- 44.009 g/mol: one carbon at 12.011 plus two oxygens at 15.999 each. By mass, CO2 is 27.29% carbon and 72.71% oxygen.
- Why does co2 in lowercase give an error?
- Element symbols are case-sensitive: one capital letter, then at most one lowercase letter. Co is cobalt and CO is carbon plus oxygen, so the calculator refuses co2 rather than guess. Co2 would be two cobalt atoms, 117.8664 g/mol; CO2 is 44.009 g/mol.
- Why does NaCl show only two decimals?
- The total carries the decimals of the atomic weight with the fewest decimals in the formula. Chlorine’s is 35.45, so NaCl is 58.44 g/mol and HCl 36.46 g/mol; digits past the second decimal would not be supported by the data. Water, with 1.008 and 15.999, shows three: 18.015 g/mol.
- How do I enter a hydrate like CuSO4·5H2O?
- Type a middle dot or a period between the parts: CuSO4·5H2O or CuSO4.5H2O. The number after the dot multiplies that part, so 5H2O adds 5 × 18.015 g/mol. Copper(II) sulfate pentahydrate is 249.68 g/mol, against 159.60 g/mol for anhydrous CuSO4.
- Why is my textbook value slightly different?
- For hydrogen, carbon, nitrogen, oxygen, sulfur, chlorine and 8 other elements, IUPAC’s CIAAW publishes an interval, not one number, because the isotope mix varies in nature. The calculator uses the CIAAW abridged values (S 32.06, Cl 35.45, Ar 39.95), so an older table that prints more digits, or a different total rounding, can differ in the last place.
- Can it handle brackets and complex ions?
- Yes, round and square brackets nest, and a number after a closing bracket multiplies the group: K4[Fe(CN)6] is 368.346 g/mol and (NH4)2SO4 is 132.13 g/mol. Charges are not accepted: enter NH4Cl rather than NH4+.
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