How to Calculate Molarity (Formula and Practice)

Molarity is one of the first calculations in Honors and AP Chemistry that combines two ideas at once, moles and volume, and that combination is exactly where most mistakes happen. Once the formula and its units are locked in, the calculations themselves are usually simple.

The molarity formula

Molarity (M) equals moles of solute divided by liters of solution: M = mol / L. The unit itself, mol/L, is often written as just "M." Notice that the volume in the denominator is the volume of the entire solution, not just the volume of solvent added to it.

Worked example: starting from grams

Problem

What is the molarity of a solution made by dissolving 20.0 g of NaOH (molar mass 40.0 g/mol) in enough water to make 500. mL of solution?

Step 1: convert grams to moles

20.0 g ÷ 40.0 g/mol = 0.500 mol NaOH

Step 2: convert mL to L

500. mL = 0.500 L

Step 3: divide moles by liters

M = 0.500 mol ÷ 0.500 L = 1.00 M

Dilutions: M1V1 = M2V2

Diluting a solution adds solvent, which increases volume but doesn't change the number of moles of solute present. That's the entire idea behind the dilution equation, M1V1 = M2V2. Whatever units you use for volume just need to match on both sides, they don't need to be liters specifically.

Where students actually lose points here

Using milliliters instead of liters

The molarity formula needs volume in liters. Plugging in 500 instead of 0.500 gives an answer that's off by a factor of 1000, and it's the single most common molarity mistake.

Forgetting the molar mass conversion step

When a problem gives grams instead of moles, skipping the conversion to moles and dividing grams directly by liters gives a number that isn't molarity at all.

Using solvent volume instead of total solution volume

Molarity always uses the volume of the finished solution, solute and solvent together, not just the water that was added.

Want help with this topic? Join live group classes for $525 per month, or book one on one sessions with Faridah for $150 per hour. Message us to book.

Molarity shows up everywhere after this

Once molarity clicks, it reappears in titrations, dilutions, and equilibrium calculations for the rest of the year in both Honors and AP Chemistry.

7+Years Tutoring

500+Students Taught

65%Score A 5

See how the program works →

Common questions

What's the difference between molarity and molality?

Molarity divides moles of solute by liters of solution. Molality divides moles of solute by kilograms of solvent. Molality is used instead of molarity when temperature changes are involved, since volume changes with temperature but mass doesn't.

How do you calculate a dilution?

Use M1V1 = M2V2, where M1 and V1 are the concentration and volume before dilution, and M2 and V2 are after. Solve for whichever value is unknown.

What if a problem gives you grams instead of moles?

Convert grams to moles first using the substance's molar mass, then divide by the volume in liters to get molarity.