Question

A set of four 15.0-mL solutions—each in a similar open dish—was left at room temperature, and the amount that remained was recorded after every hour. Which of the solutions do you expect to be the first to completely evaporate from the open dish?
1.5 m NaOH
1.0 m NaOH
0.5 m Na2SO4
0.25 m Na2SO4

Answer 1

Using Raoult's law, we know that the vapour pressure of a solution is the mole fraction of the solvent.
The mole fraction of solvent increases as the mole fraction of solute decreases.  For the present case, the mole fraction of the solvent increases as the number of moles of the solute decreases.

We also know that the higher the solvent vapour pressure, the faster it will dry out, everything else being equal (e.g. presence of drafts, temperatures, etc).

So the fastest evaporating solution is the one with the least mole of solute, namely 0.25 M Na2SO4, since this has the lowest molarity among the four solution, and hence the highest solvent vapour pressure.

Note: This question does not require math, but more chemistry is involved.  Posting in the right section will net you correct answers faster, in the future.

Answer 2

0.25 m Na2SO4 just finished the test