Calculating Molality

Known                                                        Unknown

Mass of water                                           M = ? mol / kg

Mass of Solute

 

Use molar mass to calculate the number of moles of solute.

15.4g NaBr X 1 mol NaBr / 102.894g NaBr

15.4 X 0.0097

0.1496 mol NaBr

Convert the mass of H2O from grams to kilograms.

125.0g H2O X 1kg H2O / 1,000g H2O

125.0 X 0.001

O.125kg H2O

Substitute all known values into the expression for molality and solve.

Molality (m) = Moles of Solute / Kilogram of Solvent

Molality (m) = 0.1496 mol NaBr / O.125 kg H2O

1.1968 mol / kg

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