What is "ppm" when dealing with concentrations?

Remember that ppm is parts-per-million and is like percentage except that it is based on 1 million parts (not 100). So by weight
ppm = (weight of solute / weight of solution) x 106
In dilute aqueous solutions, the specific gravity is approximately 1 and therefore 1 ppm = 1 mg/L. So CHANGE ppm to mg/L for all our calculations. Then you can convert that to mol/L to get molarity terms (molar solubility).