# Molar mass/weight

Just as understanding atomic mass, it is vital for an aspiring chemistry student to grasp the concepts of amount of substance and molecular mass/weight. Once you’ve understood and mastered them, you will find that they are extremely convenient when it comes to practical chemistry.

## Amount of substance. The concept behind mole.

Amount of substance is what it actually sounds like: an actual number of elementary particles, which the particular substance is comprised of. That’s right, if we are speaking about a stuff made of molecules, like water for example, than amount of substance is a number of water molecules in a given sample.

Since atoms, ions and molecules are so tiny, expressing amount of substance in actual number of constituent particles would not be so pretty: you would get numbers with at least 20 zeros! Therefore a reference unit was needed and at some point it was created and called a mole. 1 mole is a number of atoms in 12 grams of carbon 12. This number equals to 6.02214 × 1023 and makes Avogadro constant (NA).

NA = 6.02214 × 1023 / mol = 6.02214 × 1023 × mol-1

So it is safe to assume that 1 mol has exactly NA particles:

1 mol = NA = 6.02214 × 1023

Thus, one mol of a substance consists of 6.02214 × 1023 particles of this substance. For the purpose of calculations in most public schools around the world, this number can be rounded down to 6 × 1023. So 2 mol will consist of 12 × 1023, (or 1.2 × 1024), 10 mol will be 60 × 1023 (or 6 × 1024) and so on.

Why exactly carbon 12 and 12 grams? It is really hard to say. Probably it is so because carbon 12 has 12 nucleons (6 protons and 6 neutrons) and we refer to 12 grams because this establishes a connection between a single tiny nucleon and one seemingly huge gram. Additionally, the mass of a neutron is a tiny bit higher than the mass of a proton, so by selecting an atom with equal number of protons and neutrons in its nucleus, we average the masses of nucleons.

## Why understanding amount of substance is important.

It is indeed a good question to ask yourself why is it important to calculate the actual number or elementary particles of a substance. You will soon find out that first of all, amount of substance can help you to evaluate the mass and/or volume of a substance (and vice versa), but more importantly, it opens a path to understanding that reactants and reaction products in chemical reactions are co-dependent and can be expressed as proportions. With the reactants and the products are often calculated in moles.

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