Molarity & Solution Preparation Calculator
Solve for molarity, solute mass, or solution volume from the other two values.
Formula: M = (mass / molarMass) / volume(L)
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What it does
The Molarity Calculator solves the concentration relationship in whichever direction you need. Give it a target molarity and volume and it returns the mass of solute to weigh out. Give it the mass you actually weighed plus the volume and it returns the real concentration. Or solve for the volume that a known mass will make up to. Molar mass is entered by hand, so the same tool works for any reagent from NaCl to a recombinant protein. It is built for preparing buffers, standards, media and stocks in teaching and research labs, and every calculation runs locally in your browser with nothing uploaded.
How it works
Molarity is moles of solute per litre of solution: M = n / V in litres, and since n = mass / molar mass, the single working equation is M = (mass / M_r) / V. The calculator only rearranges that one relationship, solving for whichever field you leave blank. Two unit conventions are fixed and worth stating plainly: mass is always taken in grams and volume is always taken in millilitres, with millilitres converted internally by dividing by 1000 before the formula is applied, and molar mass must be supplied in g/mol. Results are reported to four decimal places. Because molarity is defined against the final volume of solution rather than the volume of solvent added, the denominator is the volume after the solute has dissolved and the flask has been made up to the mark, not the volume of water you poured in.
Worked example
To make 500 mL of 0.100 M sodium chloride, with M_r of NaCl equal to 22.99 + 35.45 = 58.44 g/mol, mass = M times M_r times V in litres = 0.100 times 58.44 times 0.500 = 2.922 g. Weigh 2.922 g, dissolve it in rather less than 500 mL of water, then bring the meniscus up to the 500 mL mark. Adding 500 mL of water to the solid instead would give slightly less than 0.100 M, because the dissolved salt itself occupies volume. Running the same equation the other way, 3.500 g of glucose with M_r 180.156 made up to 250.0 mL is 3.500 / 180.156 = 0.01943 mol in 0.2500 L, or 0.0777 M, and the tool reports the intermediate mole value as 0.0194 mol alongside it. Scaling to a one litre flask triples the requirement to 5.844 g of NaCl for the same 0.100 M.
When to use it
Use it to convert any two of mass, molarity and volume into the third: weighing a buffer component, checking the true strength of a solution you made from a measured mass, or scaling a recipe to a different flask size. The most common mistake is a units one. The tool takes grams and millilitres and offers no unit selector, so a mass typed in milligrams or a volume typed in microlitres is taken at face value and returns a concentration wrong by a factor of a thousand. The second is entering the wrong molar mass, and nothing checks it. Using the atomic weight of sodium, 22.99, instead of the formula weight of NaCl, 58.44, for a 0.100 M, 500 mL preparation asks for 1.150 g instead of 2.922 g, about 61 percent too little, and the solution actually comes out at 0.039 M. Hydrates catch people out in the same way: CuSO4·5H2O needs 249.68 g/mol, not the 159.60 of the anhydrous salt, so weighing against the wrong figure gives a solution 1.56 times too concentrated. Finally, remember that molarity is temperature dependent, because the volume of the solution changes with temperature while the moles of solute do not.
FAQ
- How do I convert grams of solute into molarity?
- Divide the mass by the molar mass to get moles, then divide by the volume in litres: M = (mass / M_r) / V(L). For 3.500 g of glucose, M_r 180.156, in 250.0 mL that is (3.500 / 180.156) / 0.2500 = 0.0777 M. Keep the units consistent: grams with g/mol, and millilitres divided by 1000 before use.
- What mass of solute do I need for a given molarity?
- Rearrange the relationship to mass = M × M_r × V(L). For 500 mL of 0.100 M NaCl with M_r 58.44: 0.100 × 58.44 × 0.500 = 2.922 g. Weigh that amount, dissolve it in part of the water, then make the volume up to 500 mL rather than adding a full 500 mL of water.
- Is molarity the same as molality?
- No. Molarity is moles per litre of solution and changes slightly with temperature as the solution expands; molality is moles per kilogram of solvent and is temperature independent. The two coincide only when the solution density is exactly 1 g/mL, so for a concentrated salt solution or a glycerol stock they should not be treated as interchangeable.
- Why does the tool insist on grams and millilitres?
- Because the internal formula fixes those units and converts millilitres to litres by dividing by 1000. There is no unit selector, so a value entered in mg, µg, µL or L is used as if it were g or mL. Convert before entering: 250 mg is 0.250 g and 50 µL is 0.050 mL. For a hydrate use the molar mass of the hydrated form, so CuSO4·5H2O is 249.68 g/mol rather than 159.60.