molarity

Molarity is a concentration: moles of solute divided by liters of solution, written M and spoken as molar. The denominator is the volume of the finished solution, not the volume of solvent poured in, which is why a solution made up to the mark in a volumetric flask reads its stated molarity and one mixed in a beaker rarely does. M = moles of solute ÷ liters of solution. A worked case: 4.00 g of sodium hydroxide, molar mass 40.00 g/mol, is 0.100 mol; made up to 250 mL, it is 0.100 ÷ 0.250 = 0.400 M.

Diluting adds solvent and leaves the moles alone, so concentration falls in step with volume. That gives the one dilution equation worth carrying: C₁V₁ = C₂V₂, where the units of C match each other and the units of V match each other — milliliters on both sides are fine. Percent by mass is the other concentration questions use, and it is solute mass over total solution mass. A milligram answer where a molarity was asked for, or a volume in milliliters left as milliliters, is the usual arithmetic slip.