How to Find Molarity

 

Molarity measures the strength of a solution. Remember, a solution is made up of two parts: the solvent (which does the dissolving) and the solute (the substance that is dissolved).  The more solute there is, the higher the concentrate and therefore the higher the molarity of the solution. Conversely, the more solvent there is, the lower the concentration and the lower the molarity.  To calculate molarity we use the equation: Molarity = moles of solute/Liters of solution. Therefore, the unit of concentration in molarity is moles/L, which can be also be called Molar (abbreviated as "M").  Molarity problems can be worded in many different ways. You must make sure that you have the correct units before plugging values into the molarity equation!

 

Example: A 100 mL solution is made with 0.2 moles of NaCl. What is the molarity of the solution? The moles of solute is given in the problem (0.2 moles of NaCl), however, the problem gives us MILLILITERS of solution when the molarity equation calls for LITERS of solution.  We therefore first have to convert the 100 mL to Liters by dividing by 1000 (since there are 1000 mL in 1 L). This gives us 0.1 L of solution.  We can then plug both values into the molarity equation: Molarity = 0.2 moles / 0.1 L = 2 moles/L. The concentration of the solution in molarity is 2 Molar or 2 M.