Understanding the Concept
The elevation boiling point method relies on the principle that adding a non-volatile solute to a solvent raises the boiling point of the solvent. The extent of this elevation is directly proportional to the molality of the solution.
The Equation
The equation that relates the boiling point elevation (ΔTb) to the molality (m) of the solution is:
ΔTb = Kb * m
Where:
* ΔTb = Change in boiling point (in °C)
* Kb = Ebullioscopic constant of the solvent (in °C kg/mol)
* m = Molality of the solution (in mol/kg)
Steps to Determine Molar Mass
1. Prepare the Solution:
* Dissolve a known weight (w) of acetic acid in a known weight (W) of a suitable solvent (e.g., water).
2. Measure the Boiling Point Elevation:
* Determine the boiling point of the pure solvent (Tb(solvent)).
* Measure the boiling point of the solution (Tb(solution)).
* Calculate the boiling point elevation: ΔTb = Tb(solution) - Tb(solvent)
3. Determine the Molality:
* You know the boiling point elevation (ΔTb) and the ebullioscopic constant (Kb) for the solvent.
* Use the equation ΔTb = Kb * m to solve for the molality (m).
4. Calculate the Molar Mass:
* Molality (m) = moles of solute / kg of solvent
* You know the weight of the solute (w) and the weight of the solvent (W).
* Rearrange the molality equation to solve for the moles of solute:
moles of solute = m * W (in kg)
* Molar mass (M) = weight of solute (w) / moles of solute
Example
Let's say you dissolve 1.00 g of acetic acid in 100 g of water. The ebullioscopic constant for water is 0.512 °C kg/mol. You measure a boiling point elevation of 0.15 °C.
1. Boiling Point Elevation: ΔTb = 0.15 °C
2. Molality:
* 0.15 °C = 0.512 °C kg/mol * m
* m = 0.15 °C / 0.512 °C kg/mol = 0.293 mol/kg
3. Moles of Acetic Acid:
* moles of acetic acid = 0.293 mol/kg * 0.100 kg = 0.0293 mol
4. Molar Mass:
* Molar mass = 1.00 g / 0.0293 mol = 34.2 g/mol
Important Notes:
* Solvent Choice: The solvent must be non-volatile and have a known ebullioscopic constant.
* Accuracy: The accuracy of this method depends on accurate measurements of temperature and weight.
* Ideal Conditions: This method assumes ideal behavior of the solution.
The actual molar mass of acetic acid is 60.05 g/mol. The result from this experiment is significantly lower due to the limitations of the method and possible experimental errors.
