What This Calculator Does
The Weight Percent to Molarity Calculator converts a solution's concentration expressed as weight percent (wt%) into molarity, measured in moles per litre (mol/L or M). Weight percent tells you how many grams of solute are present per 100 grams of solution, but laboratory work and stoichiometry usually require molarity. By combining the weight percent with the solution's density and the solute's molar mass, this tool gives you the exact molar concentration in seconds.
How to Use It
Enter three values: the weight percent of the solute (for example, 37 for 37% concentrated hydrochloric acid), the density of the solution in grams per millilitre, and the molar mass of the solute in grams per mole. The calculator returns the molarity along with the grams of solute per litre. Density values are commonly printed on reagent bottle labels, and molar masses can be summed from the periodic table.
The Formula Explained
The relationship is:
$$M = \frac{10 \times \text{wt\%} \times \text{Density (g/mL)}}{\text{Molar Mass (g/mol)}}$$
The factor of 10 arises from unit conversion. Density in g/mL multiplied by 1000 mL/L gives grams of solution per litre; multiplying by wt%/100 gives grams of solute per litre; dividing by molar mass (g/mol) gives mol/L. The constants 1000 and 1/100 simplify to the factor 10.
Worked Example
Concentrated hydrochloric acid is 37 wt% with a density of 1.19 g/mL and HCl has a molar mass of 36.46 g/mol. Then $$M = \frac{10 \times 37 \times 1.19}{36.46} = \frac{440.3}{36.46} \approx 12.08 \text{ mol/L},$$ which matches the familiar ~12 M value for concentrated HCl.
FAQ
Why do I need the density? Weight percent is mass-based while molarity is volume-based, so density bridges mass and volume.
What units should I use? Density in g/mL, molar mass in g/mol, and weight percent as a plain number (37, not 0.37).
Does this work for any solute? Yes — as long as you provide the correct molar mass and the density of the actual solution, not the pure solvent.
