Question

Which of the following would have the highest vapor pressure?

A) 1.0 M solution of ionic compound sodium chloride (NaCl)

B) 1.0 M solution of ionic compound potassium chloride (KCl)

C) 1.0 M solution of molecular compound sucrose (C12 H22 O11)
D) pure water

Answer 1

Vapour pressure is indirectly proportional to the number of solute particles present in the solution.

In option #1 there is 1.0 Moles of NaCl in 1 litre of solution. Since, NaCl dissociates into \(\sf Na^+~and~Cl^-\), 1 litre of 1.0 M NaCl solution will actually have 2 moles of particles.

Same is with 1.0 M KCl solution. When we look at option C, it has only 1 moles of particles in a litre of solution since \(\sf C_{12}H_{22}O_{11}\) doesn't dissociates. Now finally in option D there are no solute particles since it is pure water and hence the vapour pressure will be highest in it.

If someone asks you to arrange the vapour pressure in its order then it will be as following,

\(~~~~~~~~~~~~~~~~~~~\huge \boxed{\sf A=B<C<D}\)

Answer 2

Abhisar is absolutely correct. I want to add something that's kinda advanced and picky.

A molecule or an ion that is in solution has a hydration shell, which is made from water molecules that point with their opposing dipole towards the center molecule. Center ion Na+: water molecules arrange themselves pointing inwards with the negative dipole oxygen. Get it?

Now: If you were to actually measure the vapour pressure, you would find a tiny difference between NaCl and KCl.

Reason for that: In Na+, the carge is more concentrated than in K+, resulting in a stronger electric field and thus attracting more mater molecules into the hydration shell. Water molecules that are inside a hydration shell will not vaporize easily, because it takes energy to escape the electric field binding them to Na+.

Bigger picture: This of course also affects both freezing and boiling point of the solution! That's why we (in germany) put a saline solution on the Autobahn when it's below 0 °C: the saline solution will freeze at about -7 to -10 °C, making it save to drive above that. Water molecules inside hydration sheels will also freeze harder because they have to change orientation for that, again escaping the ion's electric field.