Calculate the molarity of a solution, calculate the mass or moles of solute needed to prepare a solution of a given molarity, and solve dilution problems using M1V1 = M2V2.
Molarity (M) is the most common way chemists measure concentration: the number of moles of solute dissolved per liter of total solution, M = moles of solute / liters of solution. A 2.0 M solution of NaCl means every liter of that solution contains 2.0 moles of dissolved NaCl -- and this measurement, unlike a simple ratio of solute to solvent, is defined by the volume of the ENTIRE solution, solute and solvent together.
Molarity connects directly to mole and mass calculations from earlier this semester: moles of solute = M x liters of solution, and from there, grams of solute = moles x molar mass, exactly as before.
Dilution is the process of adding more solvent to a concentrated ("stock") solution to lower its concentration, without changing the actual amount (moles) of solute present -- only the volume increases, spreading the same amount of solute more thinly. Since moles of solute stays constant through a dilution, M1V1 = M2V2 (concentration times volume before dilution equals concentration times volume after), where M1 and V1 describe the concentrated stock solution and M2 and V2 describe the final, diluted solution.
Mastery looks like: They correctly calculate molarity from moles and volume, calculate the mass or moles of solute needed for a target molarity, and solve dilution problems using M1V1 = M2V2.
Common stumbles: Dividing moles by milliliters instead of converting to liters first; assuming moles of solute change during a dilution when only the volume actually changes.