Unit IV: Solutions & Molarity

The Problem: Chemistry in the Real World

So far, we've mastered stoichiometry with pure substances and solids. But most chemistry doesn't happen with neat piles of powder. It happens in solutions—liquids with stuff dissolved in them. Your blood, the ocean, a cup of coffee—these are all solutions.

How do we apply our stoichiometry skills to reactions happening in a liquid? We can't just weigh the liquid, because most of it is just the solvent (like water). We need a way to know exactly how much of our reactant is actually dissolved in it. This is where the concept of concentration comes in.

The Solution: Molarity (M)

The most common unit of concentration in chemistry is Molarity. It's a simple but powerful ratio that tells us how many moles of a substance (the solute) are dissolved in one liter of the total solution (the solvent + solute).

This is the key! If you know the molarity of a solution and its volume, you can instantly calculate the moles of solute inside. This allows us to use all the stoichiometry skills we've already learned.

Working with Solutions: Dilution

Chemists often prepare concentrated "stock" solutions and then dilute them to a desired concentration for an experiment. The process of dilution involves adding more solvent to decrease the concentration of the solute.

The key principle is that the moles of solute do not change during dilution; they are just spread out in a larger volume. This gives us the incredibly useful dilution equation:

Where M₁ and V₁ are the initial molarity and volume, and M₂ and V₂ are the final molarity and volume. Use the simulator below to see this in action:

1. Click the "Add 0.5 L of Water" button to dilute the solution.
2. Observe how the volume increases and the color fades, indicating a lower concentration (molarity).
3. Notice that the total moles of solute remain constant throughout the process. This is the key to dilution!