Molality: Moles of Solute/kg of Solvent? Example

[r_swayze]

Is molality equal to moles of solute / kg of solvent

or is it equal to mole of particles of solute / kg of solvent?

example: is the molality of 0.5 mol of NaCl dissolved in 1 kg of water 0.5 m or 1 m?

 

[PhaseShifter]

If you're concerned about NaCl it would be 0.5 m NaCl.

You would treat it as 0.5m Na⁺ + 0.5m Cl^- if you're trying to calculate boiling point elevation or freezing point depression of the water, though. (Because you're interested in the fraction of ions/molecules that aren't solvent)

 

[Bohrok]

The first one is molality; the second one is osmolality.