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 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


SOLUTION-freezing-point-depression-calculation

ΔTb = Kb • M

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

amole

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


Learn more about molality

MOLALITY IS THE MOLES OF SOLUTE IN 1 KG SOLVENT

Step2 – Calculate the boiling point elevation
tb
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

What is vapor pressure chemistry cartoon
Atmospheric pressure affecting boiling point

Further reading on how atmospheric pressure affects boiling point.

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