Molecular Weight Calculator
Type a formula — C6H12O6, Ca(OH)2, CuSO4·5H2O — and this adds up the standard atomic weights, then breaks the result down by element so you can see where the mass actually sits.
The Formula
Brackets and hydrate dots are understood. Case matters: Co is cobalt, CO is carbon monoxide.
What the number means
Molar mass is the mass of one mole of a substance, in grams per mole, and it is found by adding the standard atomic weight of every atom in the formula. Those atomic weights are averages over the natural isotopic abundance of each element, which is why chlorine is 35.45 rather than a whole number — natural chlorine is roughly three parts chlorine-35 to one part chlorine-37. For work where isotopes matter, such as mass spectrometry, the monoisotopic mass is used instead and the two differ noticeably.
Case is not decoration
Element symbols are one capital letter, optionally followed by one or two lower-case letters. CO is carbon monoxide; Co is cobalt. Writing CO2 as Co2 asks for two cobalt atoms and gives an answer nearly three times too large, with no error message from any calculator that does not check. This one parses strictly and tells you which symbol it did not recognize rather than silently skipping it, because a silently dropped atom produces a wrong molar mass that looks perfectly reasonable.
Brackets and hydrates
A subscript after a closing bracket multiplies everything inside it: Ca(OH)2 is one calcium, two oxygen and two hydrogen. Hydrates are written with a dot — copper sulfate pentahydrate, CuSO4·5H2O, carries five water molecules per formula unit, and those waters are part of the mass. Weighing out the anhydrous mass when your bottle contains the hydrate is a classic way to end up 36% short on the active ingredient.
Percent composition is a fingerprint
Breaking the molar mass down by element gives the mass percentage of each, which historically was how empirical formulas were determined: burn a sample, measure the products, work backwards to the ratio of atoms. It also explains some counterintuitive results. Glucose, C6H12O6, contains twice as many hydrogen atoms as carbon, yet hydrogen is only 6.7% of its mass while carbon is 40%, because a carbon atom is twelve times heavier. Counting atoms and weighing them are different questions.
Empirical versus molecular formula
An empirical formula gives the simplest whole-number ratio of atoms; a molecular formula gives the actual count in one molecule. Glucose and formaldehyde share the empirical formula CH2O but have molar masses of 180.16 and 30.03 g/mol respectively. Percent composition alone cannot distinguish them — you need the molar mass as well, which is why both numbers are quoted together in analysis.
Frequently Asked Questions
Why is chlorine 35.45 and not 35?
Does capitalization matter?
How do I enter a hydrate?
What is the difference between molecular and empirical formula?
Sources
Official publications only. Links open the original document in a new tab.
- National Institute of Standards and Technology CODATA fundamental physical constants Atomic masses and fundamental constants
- National Institute of Standards and Technology The International System of Units (SI) The mole and the unified atomic mass unit