Molarity shows up constantly in intro chemistry, and the calculation itself is simple once the formula is memorized — most chemistry homework help requests on this topic come from mixing up units, not from the math itself.
The molarity formula
Molarity (M) = moles of solute ÷ liters of solution
Units matter here: if you're given milliliters, convert to liters first (divide by 1000). If you're given grams of solute, convert to moles first using molar mass.
Step-by-step method
- Convert the amount of solute to moles, if it isn't already given in moles.
- Convert the volume of solution to liters, if it isn't already given in liters.
- Divide moles of solute by liters of solution.
Molarity calculation example
What is the molarity of a solution made by dissolving 20 g of NaOH (molar mass 40 g/mol) in enough water to make 500 mL of solution?
Moles of NaOH: 20 g ÷ 40 g/mol = 0.5 mol
Convert 500 mL to liters: 500 ÷ 1000 = 0.5 L
Molarity = 0.5 mol ÷ 0.5 L = 1.0 M
Dilution problems: a common variation
Chemistry concentration problems often ask you to dilute a solution to a new concentration, using: M₁V₁ = M₂V₂ (initial molarity × initial volume = final molarity × final volume). Plug in the three values you know and solve for the fourth.
Common mistakes
Forgetting to convert mL to L before dividing. Molarity is always moles per liter — using milliliters directly gives an answer 1000 times too large.
Using the mass of solute instead of moles. Molarity is based on moles, not grams — always convert grams to moles using molar mass first.
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