Chemistry - Molarity

In chemistry, we often need to know exactly how much of a substance (the solute) is dissolved in a liquid (the solvent). To measure this precisely, chemists use Molarity (M). Molarity is defined as the number of moles of solute per liter of solution.

THE FORMULA

M = n / V
Where:
M = Molarity (units are mol/L, often abbreviated as M)
n = Number of moles of solute (measured in mol)
V = Total volume of the solution (measured in Liters, L)

IMPORTANT RULES TO REMEMBER

- Volume MUST be in Liters: If a problem gives you milliliters (mL), you must convert it by dividing by 1000. For example, 500 mL = 0.5 L.

- Grams to Moles: If a problem gives you the mass of the solute in grams, you must use the molar mass from the Periodic Table to convert it to moles first (grams divided by molar mass = moles).

PRACTICE PROBLEMS

Problem 1: What is the molarity of a solution that contains 0.75 moles of NaCl dissolved in 1.5 Liters of water?

Problem 2: A student dissolves 2.0 moles of MgCl2 in enough water to make 400 mL of solution. What is the molarity?

Problem 3: You weigh out 40.0 grams of Sodium Hydroxide (NaOH). You dissolve it in water to create a total volume of 2.0 Liters. What is the molarity of this solution? 

ANSWER KEY

Problem 1:
Work: M = 0.75 mol / 1.5 L
Answer: 0.5 M

Problem 2:
Work: 400 mL = 0.4 L. Then, M = 2.0 mol / 0.4 L
Answer: 5.0 M

Problem 3:
Work Step 1: 40 g / 40 amu = 1.0 mol
Work Step 2: M = 1.0 mol / 2.0 L
Answer: 0.5 M