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"#.
