Many chemical reactions occur in an aqueous environment. Premixed solutions of known concentration provide chemists a convenient way to accurately measure out reactants. A solution of known molarity always provides a specific amount of solute for a given volume of solution. The molarity is the number of moles of solute per liter of solution.There are however certain characteristics of substances that can only be measured based on a known amount of solvent rather than a known amount of solution. Examples of these include solubility, or the effect of solute on freezing or boiling point. Molality is a measure of concentration based on the number of moles of solute and the number of kilograms of solvent. This PowerPoint presentation describes molarity and molality, and illustrates several types of calculations that can be done with these measures. There are worksheets on this topic available free of charge on my website at www.evanschemistrycorner.com.