A 2.5 kg sample of well water holds 0.030 mg of arsenic…

Mass percent $= \dfrac{\text{mass of solute}}{\text{mass of solution}} \times 100$. Molarity is moles per litre of solution, and molality is moles per kilogram of solvent. One ppm is $1\ \text{mg}$ of solute in $1\ \text{kg}$ of solution, and one ppb is $1\ \mu\text{g}$ in $1\ \text{kg}$. The density joins the mass of a solution to its volume. For dilution, $M_1V_1 = M_2V_2$.