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ChemistryEdu Logo Solution | Raoult's Law#

Raoult's Law to calculate Vapour Pressure#

  • Raoult's Law states that the partial pressure or vapour pressure of any volatile constituent of solution at a given temperature is equal to the product of vapour pressure of pure constituent and its mole fraction in the solution.
\[p = {p^o\ Χ}\]
  • For a solution containing two volatile liquids A and B, we can write:
\[p_A = {p^o_A\ Χ_A}\]
\[p_B = {p^o_B\ Χ_B}\]

where,

pA = partial pressure of A

pB = partial pressure of B

poA = partial pressure of pure liquid A

poB = partial pressure of pure liquid B

ΧA = mole fraction of A in solution

ΧB = mole fraction of B in solution

  • From Dalton's law, we can calculate total pressure, P:
\[P = {p_A + p_B}\]
\[P = {p^o_A\ Χ_A + p^o_B\ Χ_B}\]
\[P = {p^o_A\ Χ_A + p^o_B\ (1-Χ_A)}\]
\[P = {(p^o_A-p^o_B)Χ_A + p^o_B}\]
  • The graph of Raoult's law will be a straight line because it is of the form y = mx+c.

Raoult's Law Graph from wikipedia

Image credit: https://en.wikipedia.org/wiki/Raoult's_law

Raoult's Law as a special case of Henry's Law#

  • According to Henry's law, partial pressure of a gas over the surface of a solution is directly proportional to its mole fraction dissolved in the solution and is given by:
\[p = {K_H\ Χ}\]
  • According to Raoult's law, partial pressure of a volatile liquid in gaseous form over the surface of a solution is also directly proportional to its mole fraction dissolved in the solution and is given by:
\[p = {p^o\ Χ}\]
  • From the above two equations, it can be clearly seen that there is a difference in proportionality constant between two equations. If a volatile liquid is dissolved in solution, then KH becomes po.

  • Thus, Raoult's law is the special case of Henry's law where KH = po.