molecular mass determination by boiling point increase

Dissolved substances influence the f.p. and the b.p. of a solvent; a liquid will boil more difficultly when strange particles are present in that liquid.

  1. When boiling a liquid, the molecules at the surface of that liquid have to escape.
    When strange particles of a dissolved substance are present, then the occupy a part of the total surface.
    That's how it becomes more difficult for solvent molecules to escape from the liquid: BOILING POINT INCREASE (b.p.i.)

The b[i does not depend very much of the character of the dissolved particles (it is not very important what kind of particles were dissolved, like ions, atoms, big or small molecules), but depent in particular on the amount of dissolved particles (number of moles).

Note that at these determinations we do not use the usual unit mol/liter, but mol/100 g solvent. Don't ask why! Has something to do with history.
The amount of unknown substance (for example p grammes) is dissolved in a weighed amount of known solvent (for example q grammes).
After measuring carefully the b.p. of the mixture (using for example a very accurate Beckmann thermometer), and knowing b.p. of the pure solvent, than you can know bpi.
The (unknown) molecular mass of the dissolved substance is M.
The number of dissolved particles is p/M. Thsi number of mols determines the increase.

The number of dissolved particles is p/M. This number of mols of particles determines the increase of decrease.
The number of mols of solute is proportional with the fpd or bpi.

b.p.i ≡ number of mol of solute

The real amount of solvent in practice rarely will be exactly 100 g; if you take less, then the effect will be stronger; if you take more solvent, then the effect will be less.
Suppose you take q grammes, then a correction factor is needed: 100/q

b.p.i. ≡ number of mols of solute x 100/q

To make the step to an =sign, a constant must be introduced: K*.
This constant K* is called: the molar b.p.i.
The value of K* is different per solvent and can be fount in tables.

If a substance is built up of ions, then this substance will dissociate in water in several particles (n particles)

b.p.i. = K* x number of mols of solute x 100/q · n


b.p.i. = K* · p · 100 · n
              M · q