A set of four 15 point 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?

Dissolved particles lower water’s vapor pressure, so a solution with fewer dissolved particles lets water evaporate more quickly. Assuming the salts dissociate in water, 0.25 m Na₂SO₄ produces about 0.75 moles of particles per kilogram of water (three ions per formula unit), less than the other solutions. Thus 0.25 m Na₂SO₄ is expected to evaporate first.

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