Keeping track of concentrations

When doing kinetics and equilibrium problems, especially when you're doing an actual hands-on experiment, there are a lot of different concentrations to keep track of. The key to keeping them straight is mostly careful reading and organization, but there are a couple common definitions or descriptions that can help.
Stock Concentration
This is the one that comes up most common in a lab experiment. "Stock" refers to the large samples of reagent from which smaller amounts are taken for individual experiments. When you come to lab, the big bottles or carboys of solution that are on the side benches or in the dispensing hood are "stock" solutions and should have a "stock concentration" listed on the bottle. Most data analysis in lab begins with stock concentrations.
Initial Concentration
In either kinetics or equilibrium problems or experiments, we will often come upon something called an initial concentration. "Initial concentration" is (to me at least) quite fascinating because it's one of those things that we can do on paper that's just not physically possible to do in the real world. The "initial concentration" in a problem or experiment is the concentration of reactants after mixing everything together but before any reaction is allowed to take place. It's as if there was a little "start reaction" button on the side of the beaker, and nothing reacted until we pushed that button. In the real world, as soon as reactant solutions come in contact with each other, they begin to react, so the "initial concentration" is never the actual concentration we might observe in a reaction mixture. The initial concentration is most commonly calculated as a dilution of the stock concentration.
{In some specific reactions, we can probably observe an initial concentration because either the reaction is SO slow that we can mix the reactants before any measurable reaction has occurred, or because there's some external stimulus (like light or heat) required to make the reaction start. These reactions aren't that rare, but they're not reactions that we're likely to use very often in Gen Chem.}
Partial Pressures
When we're working with gases, we can often use Molarity to express the concentration of reactants, but we can also use partial pressures of the gaseous components. Partial pressures are a way to measure the number of moles of a specific gas in a mixture of gases. {Dalton's Law of Partial Pressures} That sounds an awful lot like a concentration... As with the vast majority of data analysis in chemistry, counting moles is the key to figuring out relationships between reacting species. If we know the concentration and volume of a liquid solution, we'll probably be calculating moles at some point. If we know the partial pressure of a gas, we'll probably also be calculating moles at some point. Chemistry is all about the mole!

The most important thing to do when approaching these problems is organization. This is especially true of equilibrium problems; if we can organize the information given in the problem, we'll be much more successful.

Did you hear about the chemist who was thinking very hard about removing excess solvent from a solution? She was concentrating.