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Here's a video discussion of solution stoichiometry, including molarity (10 min): Khan Academy: Stoichiometry of a Reaction in Solution, on YouTube

Here's another video discussion of solutions and molarity, (13 min): CrashCourse Chemistry, Water and Solutions: For Dirty Laundry, on YouTube

Concentration and Molarity

Important Definitions

Concentration means how much of something there is in a given volume, kind of like density, except that it describes solutions. A solution is some compound, called the solute that is dissolved in another, more abundant compound, called the solvent. To be a solution, the molecules or ions of the solute must be separated from each other and surrounded by solvent molecules or ions. If very small bits of one compound are mixed into another compound, but not actually dissolved into molecules or ions, that is called a suspension (a solid in a liquid) or emulsion (two liquids, or I guess two solids since chocolate is an emulsion). In a solution, both the solute and solvent can be any phase, solid, liquid or gas. The concentration of a solution is the amount of solute divided by the total amount of solution, usually. However, there are many different units used for concentration, and some of them assume that there is so much more solvent than solute that you can use amount of solvent instead of amount of solution.

Units for Concentration

There are many, many units used for concentration. Some are mostly used to describe concentrated solutions (that have a lot of solute) and others are mostly used to describe dilute solutions (that have very little solute). The most common unit in chemistry is molarity (abbreviated M), which is moles of solute divided by liters of solution.

Another common unit is weight %, which means

Pitfalls to avoid: 1) If a concentration unit is given, remember that the substance is not pure, and calculations that would work on a pure substance don't apply. If you have 2 g of a 3% salt solution, most of that mass is water, not salt. 2) Molarities range from the very tiny, micromolar or less, to about 15-20 M at the very max. If you get an answer higher than that, you probably made a mistake.

Example Concentration Unit Calculations

HCl (hydrogen chloride) is a really nasty, dangerous gas, but dissolved in water it makes a convenient acid (hydrochloric acid) for many applications in the lab or in industry. When working with an HCl solutions in the lab, we often want to measure the mass of volume of solution used and know how many moles of HCl we added. Unfortunately, when you buy HCl, it usually comes as concentrated HCl, and the bottle will say something like "32% by weight, density 1.1593". Convert this to molarity.

To solve this, we need to think of it like a unit conversion. A good trick for dealing with % quantities is just to translate that into g/g, like this: 32 weight % = (32 g HCl)/(100 g solution). Now we do our usual unit conversion:

Now suppose you want to make 2.5M HCl using the 10M HCl. You will need to dilute it, which means adding solvent to decrease the concentration. If you want to make 1L of 2.5M HCl, how much 10M HCl do you dilute?

To solve this, we can still think about it like a unit conversion. We want 1L of 2.5M HCl solution. In the first step, we will convert to the number of moles of HCl needed to make that solution. In the second step, we will find how many ml of 10M solution have this number of moles HCl.

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