Boiling Point Formula – Boiling Point Elevation Formula & Solved Examples
By definition, the boiling point is the temperature at which the vapor pressure of the liquid equals the surrounding pressure and liquid turns into vapor. The phenomenon of boiling is pressure dependent and hence the boiling point of a liquid may change depending upon the surrounding pressure. For example, due to the change in atmospheric pressure at different altitude, water boils at 100 °C (212 °F) at sea level, but at 93.4 °C (200.1 °F) at 1,905 meters (6,250 ft). For a given pressure, different liquids will boil at different temperatures.
Elevation in boiling point:
We know that pure water boils at 100 °C at 1 atm pressure but an interesting thing happens to the boiling point if we add a small amount of salt to water. It has been experimentally proven that adding any form of non-volatile solute to a liquid increases its boiling point. The amount by which the boiling point changes is directly proportional to the amount of solute added.
Let T0b denote boiling point of pure liquid, and Tb denote the boiling point of solution (solute + pure liquid). Then,
ΔTbαmΔTbαm
ΔTb=Kb×mΔTb=Kb×m
Where, ΔTb=T0b−TbΔTb=Tb0−Tb is elevation in boiling point,
‘m’ is the molality,
‘KbKb’ is the molal elevation, boiling point or ebullioscopic constant whose value depends only on the solvent
The equation can further be written as,
ΔTb=1000×Kb×wM×WΔTb=1000×Kb×wM×W where, ‘’ is weight of the solute, ‘’ is the molar mass of the solute and ‘W’ is the weight of the solvent in grams.