How it works
This calculator converts the mass of a dissolved substance into molarity—the number of moles dissolved in each liter of final solution. You provide three pieces of information: how much solute you have in grams, the molar mass of that solute, and the total volume of your solution in liters. The calculator then divides the number of moles by the volume to give you molarity in mol/L (or M).
Molarity is essential in chemistry labs, pharmaceutical manufacturing, food science, and any field where precise solution concentrations matter. It tells you exactly how many molecules or ions are present in a given volume, which directly affects reaction rates, pH, and other chemical properties.
The formula
Molarity (M) = (mass in grams ÷ molar mass) ÷ volume in liters
Or equivalently: M = moles ÷ liters
Worked example
Suppose you dissolve 58.5 g of sodium chloride (NaCl) in water and dilute the solution to a final volume of 2.5 L. The molar mass of NaCl is 58.5 g/mol.
Step 1: Calculate moles of NaCl.
moles = 58.5 g ÷ 58.5 g/mol = 1 mol
Step 2: Divide moles by volume in liters.
Molarity = 1 mol ÷ 2.5 L = 0.4 M
Your solution is 0.4 molar (or 0.4 mol/L). This means every liter of your solution contains 0.4 moles of NaCl—roughly 2.4 × 10²² sodium ions and an equal number of chloride ions.
Second example: You have 25 g of glucose (C₆H₁₂O₆, molar mass 180 g/mol) dissolved in 0.5 L of water.
Step 1: moles = 25 g ÷ 180 g/mol = 0.139 mol
Step 2: Molarity = 0.139 mol ÷ 0.5 L = 0.278 M
Rounded, this is approximately 0.28 M glucose.
Common mistakes
Volume unit confusion: The most frequent error is forgetting to convert milliliters to liters. If your solution is 500 mL, that's 0.5 L, not 500 L. Always double-check your volume input.
Using the wrong molar mass: Ensure you're using the molar mass of the entire compound, not just one element. For example, the molar mass of H₂SO₄ is about 98 g/mol, not 32 (sulfur alone).
Forgetting to account for final volume: Molarity depends on the final volume of the solution after dilution, not the volume of solvent added. If you add 25 g of solute to 475 mL of water, the final volume is usually slightly more than 500 mL due to the volume occupied by the dissolved particles. For precise work, always dilute to the mark on a volumetric flask.
Rounding too early: Keep extra decimal places during intermediate steps, then round your final answer. Rounding moles before dividing by volume can introduce error.
This calculator provides an estimate based on the inputs you supply. For critical laboratory or pharmaceutical applications, verify your calculations and consult a qualified chemist.