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
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