Question

What is the boiling point of sugar solution at atmospheric pressure? How do we calculate the boiling point elevation of sugar solution?

Answer 1

Boiling point of sugar solution:

• “The vapor pressure of liquid increases with the increase of temperature. It boils at the temperature at which its vapor pressure is equal to the atmospheric pressure.”
• Let ${{\mathrm{T}}_{\mathrm{b}}}^0$ be the boiling point of pure solvent and ${\mathrm{T}}_{\mathrm{b}}$ be the boiling point of the solution. The increase in the boiling point $∆{\mathrm{T}}_{\mathrm{b}}={\mathrm{T}}_{\mathrm{b}}-{{\mathrm{T}}_{\mathrm{b}}}^0$ is known as the elevation of the boiling point.
• The elevation of boiling point $(∆{\mathrm{T}}_{\mathrm{b}})$ is directly proportional to the molal concentration of the solute in a solution. Thus,

$∆{\mathrm{T}}_{\mathrm{b}}\propto \mathrm{m}$

• Or

$∆{\mathrm{T}}_{\mathrm{b}}={\mathrm{K}}_{\mathrm{b}}\mathrm{m}$

The calculation of boiling point elevation can be calculated by using the expression for the elevation of boiling point.

For example:

Moles of sucrose = $5.843\mathrm{moles}$

m=moles of solute / kilograms of solvent =$5.483/1=5.483\mathrm{mol}/\mathrm{kg}$

Here

As sugar is non-electrolyte is 1.

K_b is $0.515°\mathrm{C}\mathrm{mol}{\mathrm{kg}}^{-1}$

m is $5.483\mathrm{mol}{\mathrm{kg}}^{-1}$

$$\begin{gathered} {\mathrm{\Delta T}}_{\mathrm{b}}=1\times 0.515\times 5.483=2.824°\mathrm{C} \\ {\mathrm{T}}_{\mathrm{b}}={\mathrm{T}}_{\mathrm{b}}°+{\mathrm{\Delta T}}_{\mathrm{b}}=100+2.824=102.824°\mathrm{C} \end{gathered}$$

The Boiling point of sugar is $102.824°\mathrm{C}$.