Molarity is the standard way to express concentration in chemistry labs. Enter the mass of solute, its molar mass and the final volume of the solution to get the molarity.
Enter the mass of solute in grams.
Enter its molar mass and the solution volume in mL.
Read the molarity.
Moles: n = mass ÷ molar mass
Molarity: M = n ÷ V (litres)
Molarity is the most common way chemists state how strong a solution is. It counts moles of dissolved substance in each litre of the final solution, written mol/L or M. This calculator takes the mass of solute in grams, its molar mass and the final solution volume in mL, and works out the moles and the molarity for you.
In school and college labs across India, reagents are labelled as 0.1 M HCl or 1 M NaOH, and titration results, reaction rates and equilibrium constants are all written in molar units. Class 11 and 12 chemistry, JEE and NEET problems use molarity constantly. The calculator answers a practical question too: if you dissolve this many grams and make it up to this volume, how concentrated is the solution?
1. Weigh the solute and write the mass in grams.
2. Find the molar mass in g/mol by adding the atomic masses in the formula, for example NaCl = 22.99 + 35.45 = 58.44.
3. Convert mass to moles: n = mass ÷ molar mass.
4. Convert the final solution volume from mL to litres by dividing by 1000.
5. Divide: molarity M = n ÷ V (litres).
6. To prepare a solution instead, rearrange: mass needed = M × V (litres) × molar mass.
Molarity is defined per litre of solution, not per litre of solvent. When you dissolve a solid, the volume usually changes a little, so adding 5.85 g of salt to exactly 250 mL of water does not give exactly 250 mL of solution. That is why labs use a volumetric flask: you dissolve the solid in some water, then add water up to the calibration mark. The volume you enter should be the volume at that mark, which is the true solution volume.
Molality (m) counts moles per kilogram of solvent. Because mass does not change with temperature but volume does, molality is preferred for colligative properties like boiling-point elevation. Normality counts gram-equivalents per litre and depends on the reaction; 1 M H₂SO₄ is 2 N in acid-base reactions because each molecule gives two H⁺ ions. Mass percent and ppm express concentration by mass. The calculator gives molarity only, which is the unit most exam questions and lab labels use.
Because volume expands slightly when warm, a solution made at 35 °C is a touch less concentrated in molar terms when it cools to 20 °C, though the change is usually small. A bigger source of error is the formula of the solute. Copper sulphate is often sold as the pentahydrate CuSO₄·5H₂O with molar mass about 249.7 g/mol, not 159.6 g/mol for the anhydrous salt. Using the wrong one makes the molarity wrong by more than 50%.
Ananya, a Class 12 student, dissolves 4 g of sodium hydroxide (molar mass 40.00 g/mol) and makes it up to 500 mL in a volumetric flask for a titration.
Moles of solute: n = mass ÷ molar mass = 4 ÷ 40 = 0.1 mol
Volume in litres: 500 mL ÷ 1000 = 0.5 L
Molarity = moles ÷ litres: = 0.1 ÷ 0.5 = 0.2 mol/L
Answer: Molarity 0.2 M; Moles of solute 0.1 mol
Dividing by millilitres instead of litres, giving an answer 1000 times too large.
Using the volume of water added instead of the final volume of the solution.
Using the anhydrous molar mass for a hydrated salt, or the other way round.
Confusing molarity (per litre of solution) with molality (per kilogram of solvent).
Weighing hygroscopic solids like NaOH slowly in open air, so they absorb water and the true mass is less.
Preparing standard solutions for titrations in school and college labs.
Solving Class 11 and 12 concentration problems and JEE and NEET questions.
Checking the strength of reagents in pharmaceutical and industrial quality control labs.
Making nutrient and buffer solutions for biology experiments.
Converting a concentration given in g/L into mol/L.
What is the difference between molarity and molality?
Molarity uses litres of solution; molality uses kilograms of solvent and does not change with temperature.
How do I make a solution of a given molarity?
Mass needed = molarity × volume in litres × molar mass.