Molecular Weight Calculator (Molar Mass)

Term Definitions

Molecular weight, molar mass, molecular mass and atomic weight are closely related and often used loosely as if they were the same thing. In practice the numbers line up: water's molecular weight is about 18.015, its molar mass is 18.015 g/mol, and an average water molecule has a mass of 18.015 daltons. The differences lie in what each term measures and in its units, as the sections below explain. All are based on the unified atomic mass unit (u), also called the dalton (Da), defined as one-twelfth of the mass of a carbon-12 atom, about 1.66054 × 10⁻²⁷ kg.

Atomic Weight

The atomic weight of an element, formally its relative atomic mass, is the average mass of its atoms compared with one-twelfth of the mass of a carbon-12 atom. It is an average because most elements occur naturally as a mixture of isotopes. Chlorine, for example, is about 76% chlorine-35 and 24% chlorine-37, which is why its atomic weight is about 35.45 rather than a whole number.

Isotope proportions vary slightly depending on where a sample comes from, so the International Union of Pure and Applied Chemistry (IUPAC) reviews the values regularly. For elements such as hydrogen, carbon and oxygen it now gives an interval rather than a single number, along with a conventional value for everyday calculations. Elements with no stable isotopes, such as technetium and the transuranium elements, have no standard atomic weight; the mass number of their longest-lived or best-known isotope is used instead.

Molecular Weight (relative molecular mass)

The molecular weight of a compound is the sum of the atomic weights of all the atoms in its formula. Like atomic weight it is a ratio to the carbon-12 standard and so has no units. For water, H₂O, it is 2 × 1.008 + 15.999 = 18.015. For ionic compounds such as sodium chloride, which form crystal lattices rather than discrete molecules, the same sum is called the formula weight.

Molar Mass

Molar mass is the mass of one mole of a substance, expressed in grams per mole. Because the mole and the dalton were defined together, the molar mass in g/mol is numerically equal to the molecular weight; one mole of water has a mass of 18.015 g. Since the 2019 redefinition of the SI the match is no longer exact by definition, but the difference is less than one part in a billion, far below what matters in a laboratory. Molar mass is what you use to convert between grams weighed on a balance and moles in a chemical equation.

Molecular Mass

Molecular mass is the mass of one individual molecule, measured in daltons. Unlike molecular weight it can refer to a specific combination of isotopes: a water molecule made of hydrogen-1 and oxygen-16 has a mass of about 18.011 Da, while one containing deuterium is heavier. Mass spectrometry measures these individual molecular masses.

Calculate Molecular Weight and Molar Mass

To find a molecular weight by hand, count the atoms of each element in the formula, multiply each count by that element's atomic weight, and add the results. Subscripts apply to the symbol directly before them. A subscript after brackets multiplies everything inside: Ca(OH)₂ contains one calcium, two oxygen and two hydrogen atoms. In a hydrate such as CuSO₄·5H₂O, the number in front of the water multiplies the whole water unit.

Example: sulfuric acid, H₂SO₄. Hydrogen 2 × 1.008 = 2.016; sulfur 1 × 32.06 = 32.06; oxygen 4 × 15.999 = 63.996. The total is 98.072, so the molar mass is about 98.07 g/mol. Hydrogen makes up 2.016 ÷ 98.072 = 2.06% of the mass, sulfur 32.69% and oxygen 65.25%.

When entering formulas, capitalize element symbols correctly: Co is cobalt, while CO is carbon monoxide.

Table of abridged standard atomic weights

Atomic weights used by this calculator, based on IUPAC's abridged standard atomic weights. Values in brackets are the mass numbers of a long-lived isotope for elements with no stable isotopes.

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