Question

Fischer Esterification experiment:

7. Using Raoult's and Dalton's laws, explain why an aqueous NaCl solution will have a higher boiling point than pure water.

Answer 1

At the boiling point of a liquid, the vapor pressure of the system equals to the pressure of the atmosphere.

According to Raoult's law, the partial pressure of a volatile components in a mixture is equal to its mole fraction multiplied by the vapor pressure of pure liquid.

Given by, Pa = Po (Xa)

where,

Pa = partial pressure of volatile component

Po = vapor pressure of pure liquid

Xa = moles fraction of volatile component in mixture

Now as we add NaCl, the mole fraction of volatile component would reduce, lowering down the partial pressure of the volatile component. Thus, we would require higher temperature to reach the pressure of atmosphere, that is boiling point (100 oC). Thus, an aqueous solution of NaCl will have a higher boiling point than pure water.

According to Dalton's law, the total pressure of a system equals the presure of individual components. As we add NaCl it forms Na+ and Cl- in aqueous system. Since the partial pressure of aqueous NaCl solution reduces, more heat is required to bring it to equalisation with total pressure. Also, the Na+ and Cl- have strong interactions with the polar water molecule, thus reducing the rate with which water can evaporate from the solution. Overall the need for higher temperatures to reach the atmospheric pressure.