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Weight Percent to Molarity and Molality Calculator

Suppliers state concentration in weight percent, but reactions and titrations need molarity. This calculator converts between them using density and molar mass, and also shows how many kilograms of active chemical sit in your tank or drum.

Your numbers

Weight percent (w/w)

Solution density

g/mL

Density at the stated temperature, from the SDS or certificate of analysis.

Molar mass of solute

g/mol

Sulfuric acid 98.08, HCl 36.46, NaOH 40.00, nitric acid 63.01.

Volume of solution

Your results

Molarity

18.385

mol of solute per litre of solution.

Active chemical in volume

1,803.2

kg of pure solute.

Molality

499.5922

mol of solute per kg of solvent.

Mass of solution

1,840

kg, since g/mL equals kg/L.

Total moles of solute

18,384.99

Density changes with temperature and concentration. Use the density at your working temperature for accurate results.

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How to use it

Enter the assay in weight percent, the solution density and the molar mass of the dissolved chemical. The defaults show concentrated sulfuric acid, which comes out near 18.4 mol/L. Add the volume to get the mass of solution and the pure chemical it contains, useful for inventory, reporting thresholds and charge calculations.

The formulas

Molarity = (wt% x 10 x density) / molar mass, where density is in g/mL. The factor of 10 converts percent and mL to a litre basis. Molality is moles of solute per kilogram of solvent, so it uses the solvent fraction (100 minus wt%). Molality does not change with temperature, which is why it is preferred for thermodynamic work.

Molarity depends on temperature through density
Molality depends only on masses
Active kg = solution kg x wt% / 100
Normality equals molarity times equivalents per mole

Common mistakes

People often use the density of pure water, which gives a wrong molarity for concentrated reagents. Another error is using volume percent where weight percent is intended; the two differ for most solutions. Check whether a label gives w/w, w/v or v/v before calculating.

Use the molar mass of the dissolved species, not the hydrate unless stated
Hydrated solids need their water counted in the molar mass
Do not apply this to mixtures of several solutes
Record temperature next to density in your lab notebook

Frequently Asked Questions

What is the difference between molarity and molality?

Molarity is moles per litre of solution and changes with temperature. Molality is moles per kilogram of solvent and does not. Molality is useful when comparing mixtures at different temperatures.

Why is the maximum 99.9 percent?

At 100 percent there is no solvent, so molality is undefined. The calculator caps the input to avoid a meaningless result.

Can I use w/v percent?

Not directly. Weight per volume percent is already grams per 100 mL, so molarity equals w/v percent x 10 / molar mass. Enter density 1.00 only if you convert that way.

Where do I find density?

The SDS section on physical properties or the supplier certificate of analysis states density, often at 20 deg C. Handbook tables also list density by concentration for common acids and bases.

If your item master holds assay and density for each chemical, Netray can help you surface it inside planning and costing screens. Ask for a quick review.

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