Question

Among \[0.1M\] Solutions of $NaCl$ , $CaC{l_2}$ and $A{l_2}{(S{O_4})_3}$ , The one with the highest vapour pressure is:
A. $NaCl$
B. $CaC{l_2}$
C. $A{l_2}{(S{O_4})_3}$
D. all have the same vapour pressure

Answer 1

Hint:
Vapour pressure will be lowered if there is more solute than there are solvent molecules.
This can be done by two methods,if the concentration is varied, higher concentration of solute will have more lowering in vapour pressure or, if more number of solute ions are present, then also the lowering will be high.

Complete step by step answer:
Let us understand the terms given in the question,
Vapour pressure of a liquid can be defined as, when a solution is placed in a closed container. Then the molecules of the liquid may exert some pressure on the container walls.
This value would be more for volatile liquids, which can be easily evaporated. Such as acetone.
This value is lower for non-volatile liquids.
Now, what happens is, when we dissolve a solute, then the solute molecules take up space between the solvent molecules. This will either be less or more than the original interaction between the solvent molecules.
Since, we already know from the question that all the solutes have the same concentration.
The only changing factor is the number of ions.
Let us see the disassociation of the molecules:
$NaCl \to N{a^ + } + C{l^ - }$
Here, we have a total of 2 ions
$CaC{l_2} \to C{a^{ + 2}} + 2C{l^ - }$
Here, we have a total of 3 ions
$A{l_2}{(S{O_4})_3} \to 2A{l^{ + 3}} + 3SO_4^{ - 2}$
Here, we have a total of 5 ions

So, we can say, for every one molecule of solvent the number of solute ions will be:
$NaCl = 2$ ions
$CaC{l_2} = 3$ ions
$A{l_2}{(S{O_4})_3} = 5$ ions
So one mole of Aluminum sulphate will have a maximum number of ions for one mole of solvent.
Hence option A is correct.

Note:In a pure solvent system there will be a lot of interaction between the solvent molecules. This value will be considered as the maximum value for interaction. However, when the solute is added, the solute molecules take up space between the solvent molecules. This reduces the interaction between the solvent molecules. New interactions are formed between the solute and solvent molecules. This is the reason for lowering of vapour pressure.