Boiling Point Elevation
The RAISING of the normal boiling point of a liquid by adding a non-volatile substance to it.
- When you add non-volatile solute to a pure solvent, the boiling point of the liquid solvent is elevated. This means that the solvent will now only boil at a higher temperature.
- The reason is because the non-volatile substance or solute becomes a hindrance like a weight pressing down. 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.
- In order for the solution to boil, the vapor pressure (upward pressure) has to reach an equal force to the atmospheric pressure (downward pressure).
- Thus heating the solution to a higher temperature is necessary to bring the solvent to a boiling point when you have to overcome both solutes and atmospheric pressure.
FORMULA: BOILING POINT ELEVATION
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
- So you can also calculate or 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)
Step1 – Calculate the moles of glucose
Moles of glucose = Mass / Molar Mass
= 30.0 / 180 = 0.167 moles
CALCULATE THE MOLAR MASS OF SUCROSE
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 |
Step2 – Calculate the molality of glucose
Molality of glucose = moles of glucose/mass of water
= 0.167 / 0.600 = 0.28 molal
Step2 – Calculate the boiling point elevation
ΔTb = Kb • M
= 0.52 x 0.28 = 0.146°C
So, the water with the glucose will boil at 100.146°C (100+0.146)
HOW ATMOSPHERIC PRESSURE AFFECTS BOILING POING
Further reading on how atmospheric pressure affects boiling point.