Weight Percent to Molarity Calculator
Convert a % w/w (weight/weight) concentration into molarity from the solution density and the solute’s molar mass — the usual step for concentrated reagents.
How it works
Molarity = ( 10 × %w/w × density(g/mL) ) / molar mass(g/mol)
Concentrated liquid reagents are sold by weight percent, but reactions are run in molarity, so the two must be linked. A litre of solution weighs its density × 1000 grams; the weight fraction tells you how much of that mass is solute, and dividing that solute mass by the molar mass gives moles per litre. The factor of 10 folds together the mL-to-L and percent-to-fraction conversions.
Worked example
Concentrated hydrochloric acid is 37 % w/w with a density of 1.19 g/mL and HCl’s molar mass is 36.46 g/mol. Molarity = (10 × 37 × 1.19) / 36.46 = 440.3 / 36.46 = 12.1 mol/L — the familiar “12 M” stock.
Sources: Molar concentration — Wikipedia
Frequently asked questions
What is the difference between % w/w and % w/v?
Weight/weight (% w/w) is grams of solute per 100 g of solution and needs the density to convert to molarity. Weight/volume (% w/v) is grams per 100 mL and converts to molarity without the density.
Where do I find the solution density?
It is printed on the reagent bottle or in a table for that concentration. Density changes with concentration, so use the value for the actual % w/w, not pure water’s 1.00 g/mL.
Why is the answer so much larger than the percent?
Because a litre of a dense concentrated solution contains a large mass of solute. The molar mass then converts that mass into a large number of moles — over 12 for concentrated HCl.
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This calculator is for educational purposes. Double-check important results before acting on them.