This Molecular Weight Calculator finds the average molecular weight, the elemental composition, and the mass distribution of a molecule given by its chemical formula.
The chemical formula should be entered using the upper case for the first character in the element’s name and the lower case for the second character (compare: Co – cobalt and CO – carbon monoxide). Indices should be entered as normal numbers after the appropriate elements or groups, e.g. H2O for a water molecule.
Parentheses ( ), square brackets [ ] and braces (curly brackets) { } can be used in the formulas. Nested brackets are also allowed, e.g. [Co(NH3)6]Cl3. The degree of nesting is unlimited but all the brackets should be balanced.
You can enter a formula manually or paste the formula copied from a web page or text document (including DOC or PDF file). Indices denoted using <sub> and </sub> html tags (e.g. H2O) as well as denoted using the ‘tiny’ numbers, like ₂ or ₅ , (e.g. H₂O) are supported and automatically converted to normal form.
Calculating Molecular Weight
Molecular weight, also called molecular mass, is the mass of a molecule of a substance, based on 12 as the atomic weight of carbon-12. It is calculated in practice by summing the atomic weights of the atoms making up the substance’s molecular formula. The calculation is based on the isotopic composition of the elements. Mass deficiency due to chemical bonding is not considered.
Molecular mass (molecular weight) is expressed in the unified atomic mass units (u) or daltons (Da). One Da is equal to 1/12 the mass of one atom of carbon-12. Conversely, the mass of one atom of carbon-12 is equal to 12 Da.
The molar mass of a chemical compound is defined as the mass of a sample of that compound divided by the amount of substance in that sample, measured in moles. It is the mass of 1 mole of the substance or 6.022×1023 particles, expressed in grams (g/mol). Since, by definition, one mole is the amount of substance in 12 grams of carbon-12, the molecular weight can be expressed by the same number but in units g/mol instead of Da. So, in case of carbon-12 we can say that its molecular (actually atomic) weight is 12 g/mol.
When calculating the mass of a molecule in our Molecular Weight Calculator we use the chemical elements’ atomic weights based on the isotopic composition of the elements. For example, the standard atomic weight of carbon is 12.011 g/mol, not 12.00 g/mol. This is because naturally occurring carbon is a mixture of the isotopes 12C, 13C and 14C which have masses of 12 Da, 13.003355 Da, and 14.003242 Da respectively.
Related calculators
Check out our other chemistry calculators such as Mole Fraction Calculator or Mass to Mole Calculator.