# What is the meaning of "parts per million concentration"? If the concentration is "250 ppm" with respect to NaCl, what are the concentrations of sodium and chloride ions?

Aug 21, 2017

$1 \cdot \text{ppm} \equiv 1 \cdot m g \cdot {L}^{-} 1$

#### Explanation:

With these dilutions, normally we do not have to consider densities of the solution versus the solvent.

And thus a $250 \cdot \text{ppm}$ solution represents $250 \cdot m g$ dissolved in $1 \cdot L$ of water. And thus $0.250 \cdot g$ is dissolved in one litre of water.

Given $1 \cdot \text{ppm} \equiv 1 \cdot m g \cdot {L}^{-} 1$, there are $1000 \times 1000 \cdot m g$ WATER, i.e. $999 , 999 \cdot m g$ per milligram of solvent, hence $\text{parts per million}$.

Of course this value represents the concentration of $N a C l$ NOT of $C {l}^{-}$ ion. Since chlorine constitutes 61% by mass of the solute, we may need to modify the given values appropriately.