# What is the most important unit of concentration in chemistry?

May 23, 2016

Of course, it is context dependent, however, most would accept that molarity is the important unit of concentration.

#### Explanation:

$\text{Molarity}$ $=$ $\text{Amount of substance (moles)"/"Volume of solution (Litres)}$.

And thus $\text{Molarity}$ has units of $m o l \cdot {L}^{-} 1$. Molarit can be dependent on temperature; a cold solution can be denser than a hotter one. Sometimes chemists use the $\text{molality}$ definition, where,

$\text{Molality}$ $=$ $\text{Amount of substance (moles)"/"Mass of solvent (kg)}$.

This concentration is temperatre independent, but for low concentrations of solute, molality and molarity are equivalent.

For much dilute concentrations, an analyst, especially one who dealt in trace quantities, might use $\text{parts per million (ppm)}$, where $\text{1 ppm}$ $=$ $\text{1 mg} \cdot {L}^{-} 1$. (This is reserved for aqueous solutions).