elevation in boiling point practical.pptx
- 1. Determination of Molecular Weight by using Elevation in boiling point Chem-3101-p
- 2. Colligative Properties • The use of colligative properties of solution in the determination of the molecular weight of various compounds has been one of the simplest methods. Since colligative properties of solutions depend only on the number of molecules of solute in them, this method is especially used for determining the molar masses of complex molecules, proteins, macromolecules, polymers. • A solution of known concentration of the substance is prepared in a solvent whose boiling point or freezing point or vapor pressure is known to us. The property to be measured is chosen in such a way that its measurement is simple and easy under the given conditions and do not vary considerably.
- 3. Boiling Point Elevation Boiling-point elevation describes the phenomenon that the boiling point of a liquid (a solvent) will be higher when another compound is added, meaning that a solution has a higher boiling point than a pure solvent. This happens whenever a non-volatile solute, such as a salt, is added to a pure solvent, such as water. When a solute is added to a solvent, the vapor pressure of the solvent (above the resulting solution) is less than the vapor pressure above the pure solvent. The boiling point of a solution, then, will be greater than the boiling point of the pure solvent because the solution (which has a lower vapor pressure) will need to be heated to a higher temperature in order for the vapor pressure to become equal to the external pressure (i.e., the boiling point). The boiling point of the solvent above a solution changes as the concentration of the solute in the solution changes (but it does not depend on the identity of either the solvent or the solute(s) particles (kind, size or charge) in the solution). Non-Volatile Solutes The boiling point of the solvent above a solution will be greater than the boiling point of the pure solvent whether the solution contains a non-volatile solute or a volatile solute. However, for simplicity, only non-volatile solutes will be considered here.
- 4. Calculations for Elevation in boiling point We know that the boiling point of a pure solvent rises when a non-volatile solute is added to it. This elevation in boiling point is proportional to the molal concentration of the solute in the solution. Mathematically it is given as: ΔTb=Kbm Where, ΔTb = elevation in boiling point Kb = Boiling Point Elevation Constant m = molal concentration of the solution we know that molality is the number of moles of solute per kilograms.
- 6. Apparatus and chemicals • Boiling point apparatus • Nonvolatile solute • Thermometer • Pipette • Beakers
- 7. Procedure Take a known quantity of solvent in the beaker and note its boiling point T1 ºC Now allow it to cool and add a known quantity of solute to it and mix to form a solution. Again, note the boiling point of solution and note the elevation/ rise in boiling point, T2 ºC Find the difference between two temperatures to find ΔTb. Put value of ΔTb in the formula to find molecular weight of given solute.
- 8. Result The molecular weight of solute is ____________ g/mole.