Question

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?

Answer 1

`1*"ppm"-=1*mg*L^-1`

Explanation:

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

And thus a `250*"ppm"` solution represents `250*mg` dissolved in `1*L` of water. And thus `0.250*g` is dissolved in one litre of water.

Given `1*"ppm"-=1*mg*L^-1`, there are `1000xx1000*mg` WATER, i.e. `999,999*mg` per milligram of solvent, hence `"parts per million"`.

Of course this value represents the concentration of `NaCl` NOT of `Cl^-` ion. Since chlorine constitutes 61% by mass of the solute, we may need to modify the given values appropriately.