Molar Mass of Water

Of the several parameters used to characterize any molecule, an important one is its molar mass. It's a concept that has evolved out of the need to have a quantitative measure of the amount of a substance used in a chemical reaction.

Water is one of the most abundant of all compounds on Earth. Its unique properties make life possible and sustain its continuity on Earth.

As you study chemistry and physics in greater detail, the single most important truth which you'll discover, is that everything is made up of molecules, which are in turn, made up of atoms. All the properties of any compound can be traced to the nature of its molecule.

To know any compound is to know its molecular structure. Today, we know a lot more about the water molecule, whose chemical name is H₂O. This molecule is made up of an oxygen atom, bonded with two hydrogen atoms.

When introducing any compound in a chemical reaction, it's essential that one measures its quantity. One way of doing it would be to simply weigh it. However, the weight of the substance cannot tell us how many molecules are contained within it. A more precise measure of the amount of any compound is its molar mass.

When you calculate the molecular weight of a substance and then express it in the unit of grams, what you get is the molar mass or 'one mole' of that substance. The molecular weight is the sum of atomic weights of the constituent atoms.

Calculation Technique

• Firstly, know the molecular formula of the compound.
• Note the standard atomic weights of each of the constituent atoms from the periodic table.

• Multiply the atomic weights of each chemical element by the respective number of atoms and take the sum of all products, which will be the molecular weight of the compound. Express the molecular weight in grams, which will be the molar mass of the molecule.

For Water
The molecular formula of water is H₂O, which tells us, that it's made up of two hydrogen and one oxygen atoms.

The standard atomic weight of hydrogen is 1.00794 and that of oxygen is 15.9994. So the molecular weight will be:

Molecular Weight of H₂O = (2 x 1.00794) + (1 x 15.994) = 18.00988

So the molar mass of water is 18.00988 gm or 0.01800988 Kg.

Heavy water is a compound which is made up of two atoms of deuterium (an isotope of hydrogen) and a single atom of oxygen, with the molecular formula being D₂O. The standard atomic weight of Deuterium (a.k.a. heavy hydrogen) is 2.01410178, while that of oxygen is 15.994.

Molecular Weight of Heavy Water (D₂O) = (2 x 2.01410178) + (1 x 15.994) = 20.02220356

Hence the molar mass of heavy water is 20.02220356 gm. Heavy water is used in nuclear reactors as a moderating medium, to slow down neutrons.