Molarity
Mass to weigh for a target concentration, or the concentration a weighed mass gives.
Result
Enter a molar mass and the known quantities.
Formula
mass = M × V × MWeff ÷ (purity ÷ 100), with MWeff = MW + n × 18.015 for n waters of crystallisation. When the PubChem entry is already a hydrate its molar mass includes the water, so the extra-waters field is disabled.
Questions
How do I calculate the grams needed for a molar solution?
Multiply the molarity by the volume in litres to get moles, then multiply by the molar mass. For 0.5 M sodium chloride in 250 mL: 0.5 × 0.25 × 58.44 = 7.31 g. Divide by the purity as a fraction if the reagent is not 100 %.
Do I use the anhydrous or hydrated molar mass?
Use the molar mass of the exact compound in the bottle. Copper(II) sulfate pentahydrate is 249.69 g/mol, not the anhydrous 159.61 g/mol. When you pick a hydrate from PubChem its molar mass already includes the water, so leave the extra-waters field at zero.
What is the difference between molarity and molality?
Molarity is moles of solute per litre of solution and depends slightly on temperature. Molality is moles per kilogram of solvent and does not. Almost all bench protocols use molarity, which is what this calculator gives.