Molarity is the standard way of describing how concentrated a solution is, and it comes up in almost every laboratory and chemistry calculation. This molarity calculator works in both directions: give it moles, volume and molar mass and it finds the concentration, or give it a target concentration and it tells you how much solute to weigh out.
What Is the Molarity Calculator?
Molarity measures the number of moles of solute in a litre of solution, and it is written with a capital M so it is not confused with the quantity of moles itself. It is the most common concentration unit in the laboratory because it scales cleanly with volume.
This calculator works in both directions. Give it the moles and the volume and it returns the molarity together with the mass concentration and the mass of solute present. Switch to the other mode and it works out how many grams to weigh out to reach a target concentration.
How the Molarity Formula Works
Molarity (M) = moles of solute ÷ litres of solution
The critical detail is that the denominator is the volume of the finished solution, not the volume of solvent you started with. Dissolving salt in a litre of water gives more than a litre of solution, and that difference matters at any reasonable concentration.
Going the other way, the mass you need is the target molarity multiplied by the volume in litres and then by the molar mass, which converts moles into grams.
Dilution and the C1V1 Rule
Diluting a solution is where molarity earns its keep. Because the moles of solute do not change when you add solvent, the concentration falls in proportion to the volume increase, which gives the relationship C1V1 equals C2V2.
That single equation answers most dilution questions. To make 500 millilitres of a 0.1 molar solution from a 1 molar stock, you take 50 millilitres of the stock and add solvent to the 500 millilitre mark. Adding 450 millilitres of water gives a little under the target because the volumes are not quite additive.
Common Molarities in the Laboratory
A 1 molar solution is made by dissolving one mole of the substance in enough solvent to make one litre of solution. Common laboratory strengths include 0.1 molar hydrochloric acid for general acid work, 3 molar sodium hydroxide for strong base, and 1 molar sodium chloride as a physiological saline.
The figures are only as good as the input, so check the molar mass of the hydrate you actually weighed. Sodium carbonate is often used as the decahydrate, and using the anhydrous figure would overstate the moles by a factor of more than three.
Frequently Asked Questions
How do I calculate molarity?
Divide the moles of solute by the litres of solution. Half a mole in half a litre is 1 molar, and 0.1 mole in 100 millilitres is also 1 molar.
What is the difference between molarity and molality?
Molarity uses the volume of the solution in litres, while molality uses the mass of the solvent in kilograms. Molality does not change with temperature, which is why it is preferred for physical chemistry work at varying temperatures.
How do I dilute a solution using the C1V1 rule?
Multiply the starting concentration by the starting volume and set that equal to the target concentration times the final volume, then solve for whichever volume you do not know. Always add solvent to the solute rather than the other way round, for safety and for accuracy.
Should I use the final volume or the solvent volume?
The final volume of the solution. When you dissolve a solid in water the volume usually increases, so adding exactly a litre of water to a solid gives slightly more than a litre of solution and therefore a slightly lower molarity than intended.
How do I make a specific volume from a stock solution?
Use C1V1 equals C2V2. For 250 millilitres of 0.2 molar acid from a 1 molar stock, multiply 1 by V1 equals 0.2 times 250 to get 50 millilitres of stock, then dilute to the final mark.