Discuss the difference between molarity and molality, state the units of each, state the symbol of each, and give an example of how each can be used when you are determining the concentration of a solution
The Correct Answer and Explanation is:
Difference Between Molarity and Molality
Molarity and molality are both measures of the concentration of a solution, but they are defined differently and used in different contexts.
Molarity, symbolized as M, is defined as the number of moles of solute per liter of solution. It is given by the formula: Molarity (M)=moles of soluteliters of solution\text{Molarity (M)} = \frac{\text{moles of solute}}{\text{liters of solution}}Molarity (M)=liters of solutionmoles of solute
Its unit is moles per liter (mol/L).
Molality, symbolized as m, is defined as the number of moles of solute per kilogram of solvent. It is given by the formula: Molality (m)=moles of solutekilograms of solvent\text{Molality (m)} = \frac{\text{moles of solute}}{\text{kilograms of solvent}}Molality (m)=kilograms of solventmoles of solute
Its unit is moles per kilogram (mol/kg).
Key Differences:
- Molarity depends on the volume of the entire solution, which can change with temperature due to expansion or contraction.
- Molality depends on the mass of the solvent, which does not change with temperature, making it useful in thermodynamic calculations like boiling point elevation and freezing point depression.
Examples:
Example of Molarity:
To prepare a 1.0 M NaCl solution, dissolve 1.0 mole of sodium chloride (about 58.44 grams) in enough water to make exactly 1.0 liter of solution. This is useful in laboratory settings where volumes are easier to measure than masses.
Example of Molality:
To prepare a 1.0 m NaCl solution, dissolve 1.0 mole of sodium chloride in 1.0 kilogram of water. This is especially useful when calculating changes in boiling or freezing points of a solution since molality is temperature-independent.
In summary, molarity and molality serve similar purposes but are chosen based on the precision and nature of the experiment.
