Molarity Calculator
Molarity from moles or from a mass and molar mass, the mass to weigh out for a target molarity, normality, osmolarity, and % w/v or mg/mL to mol/L.
Result…
How it's calculated
Molarity (mol/L) = moles ÷ litres of solution Moles = mass (g) ÷ molar mass (g/mol) Mass needed = molarity × litres × molar mass Normality = molarity × equivalents Osmolarity = molarity × particles per formula unit g/L = mg/mL = molarity × molar mass; % w/v = g/L ÷ 10
Molarity is the amount of a solute in moles per litre of the finished solution. To get it from a weighed mass you first turn grams into moles with the molar mass. Working backwards gives the mass to weigh out for a concentration you want. Normality counts reactive equivalents per litre: the H⁺ an acid gives or a base takes, or the electrons in a redox reaction, so the same solution can have more than one normality. Osmolarity counts dissolved particles per litre, assuming every ion separates completely. Percent w/v is grams per 100 mL, so 1% w/v is 10 g/L, and 10 mg/mL is the same as 10 g/L.
Your numbers, step by step
About the Molarity calculator
This molarity calculator works out the concentration of a solution in moles per litre. Start from the moles of solute, or from a weighed mass and its molar mass, and the volume of the finished solution. Or work backwards to the mass you need to weigh out for a target molarity in mol/L, mmol/L or µmol/L. Beside the molarity it gives the normality from the equivalents per formula unit, the ideal osmolarity from the particles each unit releases, and the same concentration as mg/mL and % w/v. Masses can be in grams, milligrams, kilograms, ounces or pounds, and volumes in litres, millilitres, microlitres, US fluid ounces or US gallons, converted with the factors in the NIST Guide to the SI. Students use it for homework, and lab staff use it when making up buffers and stock solutions. Every step of the working is shown.
How to use it
- Choose what to “Find”: molarity from a mass, molarity from moles, the mass needed for a molarity, or molarity from % w/v or mg/mL.
- Enter the “Mass of solute” or “Amount of solute”, or the “Target concentration”, with its unit.
- Enter the “Molar mass of solute” in g/mol. The molar mass calculator adds it up from a formula.
- Enter the “Volume of solution”, meaning the final volume after mixing, and pick its unit.
- Set the “Equivalents per formula unit” and “Particles per formula unit” to get normality and osmolarity, then read the results.
Worked example
To make 250 mL of 0.15 mol/L sodium chloride (58.44 g/mol): moles needed = 0.15 × 0.25 L = 0.0375 mol, and mass = 0.0375 × 58.44 = 2.1915 g. NaCl gives two ions, so the ideal osmolarity is 0.3 osmol/L. The same solution is 8.766 mg/mL, or 0.8766% w/v.
Updated
Questions people ask
How do I calculate molarity from grams?
Divide the mass in grams by the molar mass to get moles, then divide the moles by the volume of solution in litres. For example, 5.844 g of NaCl in 1 L is 0.1 mol, so 0.1 mol/L.
What is the formula for the mass needed to make a solution?
Mass in grams = molarity × volume in litres × molar mass. Weigh that out, dissolve it, then add solvent up to the final volume.
What is the difference between molarity and normality?
Molarity counts moles of solute per litre. Normality counts reacting equivalents per litre, so it is molarity times the H⁺, OH⁻ or electrons each formula unit supplies in the reaction. A 1 mol/L sulfuric acid solution is 2 N as an acid.
How do I convert % w/v to molarity?
Multiply the percentage by 10 to get g/L, then divide by the molar mass. A 0.9% saline is 9 g/L, which is 9 ÷ 58.44 = 0.154 mol/L.
Why should I use the final volume rather than the water I add?
Molarity is per litre of solution. Dissolving a solid changes the volume a little, so add water up to the mark of a volumetric flask instead of measuring the water first. To make a weaker solution from a stock, use the dilution calculator.
Is the osmolarity exact?
No. It counts every ion as fully separate, which is the ideal value. Real solutions behave as if they have slightly fewer particles, so a measured osmolality is usually a little lower. Use a measured value where it matters, such as in clinical or cell culture work.