Molality is a measure of concentration and it is defined as the moles of solute dissolved in 1 kg of solvent.
molality = moles of solute/(mass of solvent in kg)
We are given the molal concentration of the solution and the mass of solvent (water), using the formula we can find the number of moles of sodium bromide.
molality = 0.600 m
1000 g = 1 kg
mass of solvent = 500.0 g * 1 kg/(1000 g)
mass of solvent = 0.500 kg
molality = moles of solute/(mass of solvent in kg)
moles of solute = molality * mass of solvent
moles of solute = 0.600 m * 0.500 kg
moles of solute = 0.300 moles
We found that we have to dissolve 0.300 moles of sodium bromide in 0.500 kg of water to get a 0.600 molal solution. We are asked for the mass, so we can use the molar mass of sodium bromide to get it.
formula of sodium bromide = NaBr
atomic mass of Na = 22.99 amu
atomic mass of Br = 79.90 amu
molar mass of NaBr = 22.99 + 79.90
molar mass of NaBr = 102.89 g/mol
mass of NaBr = moles of NaBr * molar mass of NaBr
mass of NaBr = 0.300 moles * 102.89 g/mol
mass of NaBr = 30.9 g
Answer: we must dissolve 30.9 g of sodium bromide.