Chemistry / Unit 8 Chem 1550 worked example, 60 to 90 seconds
Converting Between Molarity and Molality
The idea
Converting a solution's molarity to molality means using the solution's density to find the mass of one liter of solution, subtracting the mass of the dissolved solute to isolate the mass of solvent, then dividing the known moles of solute by that solvent mass in kilograms.
In practice
A 0.556 M glucose solution with density 1.04 g/mL contains 0.556 mol glucose per liter. One liter of solution has a mass of 1040 g; subtracting the glucose's own mass, about 100 g, leaves 940 g of water. Dividing 0.556 mol by 0.940 kg gives a molality of about 0.591 m.
Where people slip
Subtracting the solvent's mass from the total solution mass, rather than the solute's mass, inverts the calculation and produces a molality with no correct meaning.
Your place on the trail
Every lesson rests on earlier ones and opens later ones. Tap any to jump.
Something wrong in this lesson? Flag it · Sources for Chemistry