Final answer:
To calculate the moles of unknown solute, you first calculate the freezing point depression, then determine the molality of the solution, and finally use the molality and mass of solvent to find the number of moles and the molar mass of the solute.
Step-by-step explanation:
To calculate the moles of unknown solute using molality and freezing point depression, follow these steps:
- Determine the freezing point depression (ΔTf) of the solution by subtracting the freezing point of the pure solvent from the freezing point of the solution.
- Use the formula ΔTf = Kf * m, where Kf is the freezing point depression constant for the solvent and m is the molality, to calculate the molality of the solution.
- Calculate the number of moles of solute present in the solution using the molality and the mass of solvent.
- Finally, divide the mass of the solute by the number of moles to find the molar mass of the solute.
This method works well for dilute solutions and is commonly used for determining the molar mass of an unknown compound when compared to boiling point elevation due to the larger value of Kf, as seen in substances like d-(+)-camphor.