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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