Question

According to Raoult's law, relative lowering of vapour pressure for a solution is equal to:

• A. mole fraction of a solute
• B. mole fraction of a solvent
• C. moles of a solute
• D. moles of a solvent

Answer 1

Answer: (A)

According to Raoult's law' the partial pressure of any volatile, component of a solution at any temperature is equal to the vapour pressure of pure component multiplied by mole fraction of the component in the solution.
$P_A=P_A^o{x}_A$
where, $P_A=$ vapour pressure of component $A$
$P_A^o=$ vapour pressure of pure component
$x_A=$ mole fraction of component $A$.
$\frac{P_A-P_A^o}{P_A^o}=x_A$
So, it is equal to the mole fraction of a solute.