Question

On analysing a sample of water, a chemist finds that 100`cm^3` of it contains 0.007g of calcium carbonate. What is the concentration of calcium carbonate in ppm?

Answer 1

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

Explanation:

And thus `"concentration"` `=` `"Mass of solute"/"Volume"`.

`=(7*mg)/(0.100*L)=70*mg*L^-1`

And thus `0.070*g*L^-1-=70*mg*L^-1-="70 ppm"`.

At these concentrations, we don't really have to worry too much about the density of the solution. `"70 ppm"` denotes a concentration of `70*mg*L^-1` with respect to `"calcium carbonate"`. Of course this will be a GREATER concentration than the concentration with respect to the metal ion. Why should this be so?

I think you can see why a concentration of `1*mg*L^-1` is referred to as a `"part per million"`. A litre of water has a mass of `1000*g`, and each of these `"grams"` is composed of a `1000*mg` (i.e. `1*mg=10^-3g`. `1000xx1000 equiv"million"`, hence `1*mg` `equiv` `"1 part per million"`.