Calculate solute needed for a solution.
Preparing a solution of known molarity is a two-step conversion: the target concentration times the final volume gives the moles needed, and the molar mass turns those moles into grams. Reagent purity then raises the mass actually weighed, because a 99 percent reagent contains 1 percent of something that is not the solute. The volume is the final volume of solution, not the volume of solvent added — the difference matters at high concentration.
Molarity to mass
moles = molarity x final volume in litres; pure mass = moles x molar mass; mass to weigh = pure mass / (purity / 100)
Laboratory calculation only. Follow your institution's risk assessment and handling procedures for the reagents involved, particularly for concentrated acids, bases and toxic solids.
Because dissolving a solute changes the total volume, sometimes substantially. Molarity is defined per litre of solution, so the correct technique is to dissolve in part of the solvent then dilute to the calibration mark of a volumetric flask.
It matters for anything below about 99.5 percent assay and for hydrated salts, where the water of crystallisation must be included in the molar mass instead. Copper sulfate pentahydrate is 249.68 g/mol, not the anhydrous 159.61.
Grams of solute per 100 ml of solution, expressed as a percentage. It is common in clinical and biological work — physiological saline is 0.9 percent w/v sodium chloride, which is about 0.154 M.