Boiling Point Elevation

Boiling point eleveation cartoon

Boiling Point Elevation

The RAISING of the normal boiling point of a liquid by adding a non-volatile substance to it.

  • When you add a non-volatile solute to a pure solvent, the boiling point of the liquid solvent is elevated due to the lowered vapor pressure. This means that the solution will boil at a higher temperature.
  • The reason is because the non-volatile substance solute decreases the number of water molecules at the surface.
  • The solvent particles will need a lot more energy to bounce upwards and push the solute particles out of the way and eventually escape into the atmosphere.

FORMULA: BOILING POINT ELEVATION

ΔTb = Kb • M•i

The equation enables you to calculate how much change in boiling temperature of a solution after adding solutes
ΔTb = change in temperature
Kb = boiling point constant (solvent)
M = molality of solutes
i = van’t Hoff factor of solute

  • You can also predict the boiling point of a solution if you know the constant(kb) and molality(M) of the solution

SAMPLE CALCULATION

Calculate the boiling point elevation of a solution containing
30.0 g of glucose (C₆H₁₂O₆) dissolved in 600 g of water.
Given Kb (constant) for water is 0.52 °C/m (per mole)

CALCULATE THE MOLAR MASS OF GLUCOSE

C6H12O6 = (12 x 6) + (1 x 12) + (16 x 6) = 180

Symbol Element Atomic weight Atoms Mass
C Carbon 12.01078 6 12 x 6
H Hydrogen 1.007947 12 1x 12
O Oxygen 15.99943 6 16 x 6

 

Step1 – Calculate the moles of glucose

amole

Moles of glucose = Mass / Molar Mass

= 30.0 / 180 = 0.167 moles

 

Step2 – Calculate the molality of glucose

Molality of glucose = moles of glucose / mass of water

= 0.167 / 0.600 = 0.28 molal


Learn more about molality

MOLALITY IS THE MOLES OF SOLUTE IN 1 KG SOLVENT

Step2 – Calculate the boiling point elevation
tb
ΔTb = Kb • M • i

For glucose (C6H12O6), the van’t Hoff factor = 1
= 0.52 x 0.28 x 1 = 0.146°C
So, the water with glucose will boil at:

(100 + 0.146 = 100.146°C)

HOW ATMOSPHERIC PRESSURE AFFECTS BOILING POING

What is vapor pressure chemistry cartoon
Atmospheric pressure affecting boiling point

Further reading on how atmospheric pressure affects boiling point.

Leave a Reply

Your email address will not be published.