Ex: [Cu] in river is 30 ppb, what is the molar concentration?

Answer 1

30 grams per billion grams of solution. Well, assuming the density of water is 1000g/liter, then

30/1E9 *1E3=30E-6 or 3E-5 parts per liter.

Now, the moles of Cu in one liter:
must be 30E-6/atomicmassCu=30/63.6 E-6

or .47E-6 moles per liter, or M=.47microMolesperliter