The relationship between M, m and n

The number of moles, n, of a substance can be found by using the following equation

n(mol) = m(g) / M(g/mol)

where m is the mass and M is the molar mass of the given substance.

The molar mass, M, is the mass in grams per one mole (g/mol or gᐧmol-1) of a substance. One mole of a substance will have the same value as the molecular mass of that substance, which can be calculated from the atomic weights of its constituent atoms. But when you look at the unit (g/mol) you can see that molar mass can also be calculated if you know the mass of a substance (g) and how many moles that corresponds to (mol):

M(g/mol) = m(g)/ n(mol)

By rearranging this equation you can also calculate how many grams, a certain number of moles should weigh, by isolating m in the equation:

m(g) = n(mol) * M(g/mol)

These three equations are very useful for many purposes in chemistry and it can be practical to find a mnemonic to memorize it. One way to do that is to write it up in a triangle like this:

The triangle with the lower row and the upper row. On the upper low there is a small letter m, on the lower row there is a big letter M multiplied by the small letter n. Next to the triangle is a picture of a bird in a sky with the small letter m covering the shape of the bird.

When you cover the item you want to calculate, you have the equation written right there. You can remember that the little m should be at the top, because it looks like a bird which fly high above everything else.