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An aqueous solution is 1.00 molal in KI. Which change will cause the vapour pressure of the solution to increase?
A
Addition of water
B
Addition of NaCl
C
Addition of $Na_2SO_4$
D
Addition of 1.00 molal KI
Detailed Solution
A non-volatile solute lowers the vapour pressure of the solvent; the lowering is proportional to the mole fraction of solute particles (Raoult's law).
Adding water dilutes the solution, so the mole fraction of solute particles decreases, the mole fraction of water increases and the vapour pressure rises.
Adding NaCl or $Na_2SO_4$ increases the number of solute particles, which lowers the vapour pressure further.
Adding more 1.00 molal KI solution leaves the concentration, and hence the vapour pressure, unchanged.
Hence the vapour pressure increases on addition of water.
Adding water dilutes the solution, so the mole fraction of solute particles decreases, the mole fraction of water increases and the vapour pressure rises.
Adding NaCl or $Na_2SO_4$ increases the number of solute particles, which lowers the vapour pressure further.
Adding more 1.00 molal KI solution leaves the concentration, and hence the vapour pressure, unchanged.
Hence the vapour pressure increases on addition of water.