Cottrell’s method for determining the boiling point elevation elevation constant.

Determination of boiling point elevation constant

The main difficult for finding molecular masses by this method arises from super heating of the liquid above its true boiling point.  This can be minimized by the following methods.

Cottrell’s method

The setup is as shown

1. Solvent/solution is heated in tube A, open tube B carries the thermometer.
2. Cottrell’s ‘pump’, C prevents over heating of the solution
3. First, the boiling point (T₀) a known mass (m_s g) of solvent is determined
4. Then a weighed solute (mass M₂ g) is introduced through the side arm and the boiling point (T₁) of the solution is determined.

 Treatment of results

Mass of solvent                                                = m_s

Mass of solute                                                  = m₂

∆t                                                                         = (T₁-T₀)

Molecular mass of solute                             = Mr

where K_b is the boiling point constant of a solvent.

Note that

This method can be used to determine the molecular mass of benzoic acid dissolved in acetone or that of naphthalene dissolved in chloroform.

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