Lesson 16: Solutions

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Now that we have a better understanding of liquids, it is time to revisit solutions and solubility. I’m sure you will be pleased to learn you will finally be finding out what the 6M in 6M HCl means.

Textbook Reading: Chapter 13, pp. 447-477. Molarity starts on page 457.

Definitions review:

We already touched a few of these concepts in an earlier lab, so it should be review.

The solute is the smallest part of a solution, or the substance being dissolved.

The solvent is the larger part, or the part doing the dissolving.

A solution is a solute dissolved in a solvent.

The text mentions temperature can have different impacts on solubility.

You probably have direct experience trying to add sugar to a cold drink versus a hot drink. Isn’t it easier to stir in sugar when the liquid is warmer? Solids in general are more soluble at high temperatures and less soluble at low temperature.

SolubilityVsTemperature(image pubic domain)

As you can see from the graph, however, not all salts follow this general trend.

Gases

Think about how quickly a warm soda goes flat. Gases in solution react just the opposite of solids. In general, gases are more likely to stay in solution at low temperatures than high ones.

Molarity

Now it is time to learn how to quantify solutions. One way is to calculate the molarity, which is the number of moles of solute per liter of solution.

molarity-equation

More review:
Do you remember what a mole of something is?

(A mole of a substance is simply Avogadro’s number or 6.022 x 1023 items of that substance.)

Do you remember how to calculate the molar mass from Lesson 7?

(Use your periodic table to find all the atomic mass units (amus) for the atoms in the molecule and then add them together and convert to grams.)

Mr. Causey walks us through the process of calculating molarity and making dilutions in this video:

Note: Some of you skip these videos, but because molarity is such a big part of doing chemistry, I really recommend you spend the time with this one.

In addition, PhET has an awesome interactive about molarity. Be sure to click the “show values” box to really see what is going on.

Please let me know if you have any questions about your readings or this lesson.

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