How to convert g/L to mol/L I know I should divide $\pu{g L-1}$ by the molar mass of the substance, but I don't seem to find the specific answer on Google. So just to be sure:
If I have $10^{-5}~\pu{g L-1}~\ce{Cu^2+}$ solution, do I have $1.57 \times 10^{-7}~\pu{mol L-1}$?
I think I'm missing something.
Yes. $$n = \frac{m}{M}~~~~~~~~~~n = cV$$
So $$c = \frac{m}{VM}$$

Given $\frac{m}{V}$ you can work out $c$ (in $\mathrm{mol~dm^{-3}}$).