Molarity, mass percent and ppm measure concentration in fundamentally different ways. This converts between them, given the solute’s molar mass and the solution’s density.
How it works
Mass percent, ppm and molarity
mass % = (mass of solute ÷ mass of solution) × 100 · ppm = mass % × 10,000
Molarity bridges to mass percent via the solution’s density, since molarity is mass-per-volume while mass percent is mass-per-mass.
Why molarity can’t convert to mass percent by algebra alone
Mass percent compares solute mass to total solution mass; molarity compares solute moles to solution volume. Converting between a mass-based ratio and a volume-based ratio needs the solution’s density to bridge mass and volume — which is why this calculator asks for it explicitly, rather than assuming a solution is as dense as pure water (1.00 g/mL), an assumption that’s only exactly true for very dilute solutions and measurably wrong for anything concentrated.
How to use this calculator
- Enter the solution’s density and the solute’s molar mass.
- Enter either a known mass percent or a known molarity.
- Read the other two concentration figures.
Frequently asked questions
What’s the difference between mass percent and ppm?
The same ratio, expressed at two different scales — 1% is exactly 10,000 ppm. Ppm is used for very dilute concentrations where a percentage would be an awkward, tiny decimal.
Why can’t I just assume the solution’s density is 1.00 g/mL?
That’s only exactly true for pure water. A concentrated solution is usually somewhat denser than water — sometimes noticeably so — and using 1.00 g/mL as a shortcut introduces real error into the molarity conversion for anything beyond a very dilute solution.
Where do I find a solution’s actual density?
A reference handbook or manufacturer’s specification sheet for that specific solution and concentration — density genuinely varies with concentration for the same solute, so a single “density of salt water” figure isn’t precise enough for anything beyond a rough estimate.
Does this work for gas concentrations too?
No — this assumes a liquid solution with a stated density. Gas-phase concentrations (like ppm in air) use a related but different relationship involving the ideal gas law instead.
Why does the molar mass matter for this conversion?
Because molarity counts moles, and moles depend on the substance’s molar mass to convert to and from a mass — without it, there’d be no way to bridge a mole-based unit and a mass-based one.
