What is Molecular Weight?

A molecule of water weighs 18 u.
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  • Written By: Georga
  • Edited By: C. Wilborn
  • Last Modified Date: 17 April 2014
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Molecular weight is simply the weight of all the atoms in a substance. A molecule is a stable group of two or more atoms and is electrically neutral. Hydrogen and oxygen, for example, are atoms, but when two hydrogen and one oxygen come together, they make H2O, or water, which is a molecule. The weight of water is determined by adding together the weight of all three atoms in a single water molecule.

Also called molecular mass, molecular weight refers to the mass of one molecule of a substance. It takes into account the average weight of the substance, where the weight of one molecule may be slightly different than another because of isotopes. Isotopes are variations of elements and have a nuclei with the same number of protons but a different number of neutrons, giving them a different atomic weight that is sometimes figured into the calculations.

Some chemists refer to this measurement interchangeably with molar mass. Molar mass refers to the weight of one mole of a substance. A mole is the amount of a substance that contains the same number of elemental entities, such as electrons, ions, and atoms, as there are in 12 grams of carbon-12. Carbon-12 is the most stable of carbon isotopes and is used as the standard by which all atomic masses are measured.


To determine the molecular weight of water, as an example, a researcher would take the weight of hydrogen and multiply it by two, since there are two hydrogen atoms in water, and add the weight of oxygen. Hydrogen weighs about 1 atomic mass unit (u) and oxygen weighs about 16 u. The calculation is 1 × 2 + 16 = 18 u, so the weight of one molecule of water is 18 u.

Mass spectronomy can also be used to determine molecular mass. It is used to determine the weight of smaller molecules, while average molecular weight is used for larger molecules. Mass spectronomy uses the mass of the most common isotope of each element. Average molecular mass is calculated based on the most common isotope of an element and often uses the weights listed on the periodic table, unless the exact isotope of a particular substance has been determined.


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Post 2

@adele14: You would probably contain the gas or substance in a closed system and keep the temperature constant. Then apply Boyle's law.

Post 1

How is weight measurable for substances that appear weightless, such as helium gas or cigarette smoke?

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