Video: Applying Knowledge of the Impact of Adding an Ionic Solute to Water on the Water’s Vapor Pressure

For statements I and II, state for each if they are true or false. I) The vapor pressure for a solution of an ionic solute will be higher than the vapor pressure of pure water. II) Ionic solutes dissociate into positive and negative ions. If both are true, state if II is a correct explanation for I.

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Video Transcript

For statements I and II, state for each if they are true or false. I) The vapor pressure for a solution of an ionic solute will be higher than the vapor pressure of pure water. II) Ionic solutes dissociate into positive and negative ions. If both are true, state if II is a correct explanation for I.

Statement I references the term vapor pressure. The vapor pressure is the pressure of a gas when a liquid is alone in a closed system at a specific temperature. The way to think about it is imagine you have a container and you pour in the liquid. At the moment, there’s a little bit of air above the liquid. So we stick on a lid and remove all that air. So we have very pure liquid in the container, but not very much gas. Once all the air is removed, some of the liquid will evaporate, filling the remaining space. The vapor pressure is the pressure of this gas.

Statement I asserts that the pressure of this gas above a solution of an ionic solute will be higher than the pressure of this gas for pure water. Let’s have a close look at the surface of the liquid for the solution and the pure water. Initially, they look quite similar. So there doesn’t seem to be a good reason why one would evaporate more than the other. But let’s look closer at the solution of the ionic solute. And for the purposes of this example, let’s assume the ionic solute is something like sodium chloride. I’ll draw the sodium ions as little pink dots and the chloride ions as little green dots.

So there we have sodium and chloride ions swimming around inside the water. How’s that going to affect the evaporation of the water? Well, the individual particles, the sodium and chloride ions, are going to interfere with water and stop it passing into the gas phase. They won’t stop it completely, but they will slow it down. The sodium and chloride ions themselves won’t evaporate. So they won’t add to the vapor pressure. So the vapor pressure for the solution should actually be lower than for the pure water. This relationship holds true for solutes that don’t evaporate easily themselves. And ionic solutes tend to have very high boiling points. So they won’t contribute to the vapor pressure.

In these circumstances, we can use Raoult’s law which tells us that vapor pressure of a solution, assuming we have nonvolatile solute, is the solvent’s mole fraction multiplied by the vapor pressure of the pure solvent. The upshot of all this is that statement I is false. The vapor pressure for a solution of an ionic solute will be lower than the vapor pressure of pure water.

However, statement two looks more promising. It states that ionic solutes dissociate into positive and negative ions. Assuming we’re dealing with water, this is indeed the case. For instance, when sodium chloride is added to water, we produce positive sodium ions and negative chloride ions. The fact that ionic solutes disassociate actually means they tend to reduce the vapor pressure of a solution much more than similar nonionic solutes. So statement II is true. But unfortunately, statement I was false. So statement II cannot be a correct explanation for I.

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