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
Density at the stated temperature, from the SDS or certificate of analysis.
Sulfuric acid 98.08, HCl 36.46, NaOH 40.00, nitric acid 63.01.
Your results
Density changes with temperature and concentration. Use the density at your working temperature for accurate results.
Add assay and density to your item master
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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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