How to Calculate Molarity: Formula, Steps, and Practice Problems

Molarity (M) is the number of moles of solute dissolved in one liter of solution. It is the most commonly used concentration unit in general chemistry because it directly connects to stoichiometry — you can go from molarity and volume to moles, then use mole ratios to solve reaction problems. The formula is M = n / V, where n is moles of solute and V is volume of solution in liters. A 1.0 M NaCl solution contains 1.0 mole of sodium chloride per liter of total solution.

Step 1: Identify the solute and find its molar mass from the periodic table. Step 2: Convert the given mass of solute to moles using n = mass / molar mass. Step 3: Convert the solution volume to liters if given in milliliters (divide by 1000). Step 4: Divide moles by liters to get molarity. Example: dissolving 11.7 g of NaCl (molar mass 58.44 g/mol) in 500 mL of solution. Moles = 11.7 / 58.44 = 0.200 mol. Volume = 500 / 1000 = 0.500 L. Molarity = 0.200 / 0.500 = 0.400 M.

When you dilute a solution by adding solvent, the amount of solute stays the same but the volume increases. The dilution equation M1V1 = M2V2 relates the initial and final molarity and volume. M1 and V1 are the concentrated solution's molarity and volume. M2 and V2 are the diluted solution's molarity and volume. Any volume unit works as long as both sides match. Example: how much 6.0 M HCl do you need to make 500 mL of 0.10 M HCl? Solve for V1: V1 = (M2 × V2) / M1 = (0.10 × 500) / 6.0 = 8.3 mL of the concentrated acid diluted to 500 mL total.

The most frequent error is using the wrong volume — molarity uses volume of solution, not volume of solvent. If you dissolve salt in water and the total solution volume is 250 mL, that is your V, not the volume of water you added. The second mistake is forgetting to convert milliliters to liters. The third is confusing moles with grams — always convert mass to moles first. Fourth, in dilution problems, students sometimes use the wrong M-V pair. Double-check which values belong to the concentrated solution and which belong to the diluted one.

Molarity problems appear on nearly every general chemistry exam. The fastest way to build speed is to practice the conversion chain: grams to moles to molarity, and the reverse. Work problems in both directions — given molarity find grams, and given grams find molarity. For dilution, practice identifying which is the concentrated and which is the dilute solution. ChemistryIQ can check your work on molarity problems from a photo of your homework, catching unit errors and calculation mistakes before you submit.