How do we know how many moles of something we have if the substance is dissolved in water? You have used many solutions in chemistry this year. The concentration of those solutions is always indicated with an "M". The "M" stands for something called molarity.

In Lesson 2: What Happens to Mass During a Chemical Reaction, you used both hydrochloric acid and acetic acid. Both of these chemical compounds were followed by (aq), meaning in an "aqueous" solution. This means that the hydrochloric acid and the acetic acid were in a solution of water.

You will need the following resources to complete this assignment.

• Molarity Calculations

Chemical reactions are typically described as so many moles of compound A reacting with so many moles of compound B to form so many moles of compound C. When we determine how much of a reactant to use, we need to know the number of moles in a given volume of the reactant. Percent solutions only tell us the number of grams, not moles. A 100 mL solution of 2% NaCl will have a very different number of moles than a 2% solution of CsCl. So we need another way to talk about numbers of moles.

Chemists need the concentration of solutions to be expressed in a way that accounts for the number of particles that react according to a balanced chemical equation. Since percentage measurements are based on either mass or volume, they are generally not useful for chemical reactions. A concentration unit based on moles is preferred. The molarity (M) of a solution is the number of moles of solute dissolved in one liter of solution. To calculate the molarity of a solution, you divide the moles of solute by the volume of the solution expressed in liters.

Note that the volume is in liters of solution and not liters of solvent. When a molarity is reported, the unit is the symbol M and is read as “molar”. For example a solution labeled as 1.5 M NH₃ is read as “1.5 molar ammonia solution”.

Your instructor will provide you with a handout so you can practice a variety of molarity calculations.