Which of the following aqueous solutions will have the lowest vapor pressure at 25°c?

1.0 m fe(no3)3
2.0 m c6h12o6
1.0 m kno3
2.0 m cacl2

Due to maximum ion concentration, CaCl₂ will have lowest vapor pressure.

Explanation:

Relative lowering of vapor pressure is a colligative property which depends upon number of solute particles in a given solution

More the number of solute particles (irrespective of nature) more the relative lowering of vapor pressure.

Let us see the number of solute particles furnished by each solute in given solution.

a) 1.0M Fe(NO₃)₃ : each solute molecule will give one ferric ion and three nitrate ions.

Thus total concentration of solute particles by 1 M is 4 M.

b) 2.0 M C₆H₁₂O₆ : this is glucose which will not dissociate in water. Thus effective concentration of glucose particles will be 2.0M only.

c) 1.0 M KNO₃: each solute will give one potassium ion and one nitrate ion.

Total concentration of particles = 2.0 M.

d) 2.0 M CaCl₂: each solute particle will give one calcium ion and two chloride ion. Thus total concentration of ions in the solution is 6.0 M.

Due to maximum ion concentration, CaCl₂ will have lowest vapor pressure.

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