To make a solution of c mol/L in a volume V litres, weigh c × V × M grams of solute and add water up to the final volume: 250 mL of 0.1 mol/L sodium chloride is 1.46 g.
Concentration is moles per litre of solution, so the moles needed are c × V and the mass is that times the molar mass: 0.1 × 0.25 × 58.44 = 1.461 g. The solute is dissolved in less water than the final volume in a volumetric flask and made up to the mark, because dissolved solute takes up room.
The volume counts the whole solution, not the water added: a litre of water with 58 g of salt dissolved in it is more than a litre. For a hydrate use its full molar mass, for an impure reagent divide by its purity, and for very exact work the temperature matters because the volume of a solution does.
Estimate with the rule, then check it against the calculator that models it properly.
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