Molality: Moles of Solute/kg of Solvent? Example

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SUMMARY

Molality is defined as the number of moles of solute per kilogram of solvent, specifically expressed as moles of solute divided by kg of solvent. In the case of 0.5 moles of NaCl dissolved in 1 kg of water, the molality is 0.5 m. However, when considering the dissociation of NaCl into its constituent ions (Na+ and Cl-), the effective concentration for calculations such as boiling point elevation or freezing point depression is 1 m, which reflects the total number of particles in solution. This distinction between molality and osmolality is crucial for accurate thermodynamic calculations.

PREREQUISITES
  • Understanding of molality and osmolality concepts
  • Knowledge of solute-solvent interactions
  • Familiarity with colligative properties such as boiling point elevation and freezing point depression
  • Basic chemistry concepts including moles and molecular dissociation
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  • Research the calculation methods for colligative properties in solutions
  • Study the differences between molality and osmolality in detail
  • Explore the impact of ionic dissociation on solution properties
  • Learn about practical applications of molality in laboratory settings
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Chemistry students, laboratory technicians, and professionals involved in solution chemistry and thermodynamic calculations will benefit from this discussion.

r_swayze
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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?
 
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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)
 
The first one is molality; the second one is osmolality.
 

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