Walk a treatment plant and every surface carries a number: a drum label, a scale readout, a lab sheet, a batch record. Half the errors in process chemistry are not arithmetic at all — they are one of those numbers being taken for another. So before any relation, learn to read the units, because the units are the only honest signature a quantity has.

Mass, $m$, is what a balance reads: grams or kilograms, with no per anywhere in it. Molar mass, $M$, is the mass of one mole of a substance, in grams per mole — a fixed property of the compound, the same on every sack of it. Amount of substance, $n$, is a count of moles: how many particles, expressed in a unit sized for a laboratory rather than for a molecule.

Molar concentration — molarity — is $C$, in mol/L: moles of solute per litre of solution. $C = \dfrac{n}{V}$, read aloud C equals n over V, where $V$ is the volume of the finished solution in litres. Mass percent is $c$, in % w/w: $c = \dfrac{m_{\text{solute}}}{m_{\text{solution}}} \times 100\%$, the solute's mass divided by the mass of everything in the vessel, times a hundred. And mass concentration is mg/L: a weight of solute in a litre of water, the currency nearly every water-quality result is reported in.

Two of those are counts per volume, one is a ratio of two masses, one is a weight per volume, and one is just a weight. They are not interchangeable and they do not convert for free: crossing from mg/L to mol/L costs you a molar mass, and crossing from % w/w to mol/L costs you a molar mass and a density.

One habit, and it will carry you through the whole chapter: read the unit before you read the number. A drum marked 12.5% and a bottle marked 12.5 mol/L have nothing to do with each other, and only one of them would survive contact with a human being.