Molarity is the first number most people get wrong in a new lab. Not because the maths is hard. It is because the bottle on the shelf rarely matches the number in the protocol.
What molarity means
Molarity (M) is moles of solute per litre of final solution. A 1 M NaCl solution holds 1 mole of NaCl, which is 58.44 g, in every litre of liquid once the volume is made up. The key words are final solution. You do not add 58.44 g of salt to 1 L of water. You dissolve it in less water and then bring the volume up to 1 L.
The smaller units scale by a thousand each time: 1 M = 1000 mM = 1,000,000 µM. Most working solutions in molecular biology sit in the mM and µM range. Stocks usually sit at 0.5 M to 5 M.
The one formula you need
Everything else is a rearrangement. If you have the mass and want the molarity, divide by volume and MW. If you have a target molarity and a fixed mass, solve for volume. Keep the units in litres and mol/L while you calculate, and convert to mL or mM only at the end. Most slips happen in the unit change, not in the multiplication.
Want more options, saved recipes and warnings? Open the full Molarity & Dilution calculator in BenchMate.
Worked examples
500 mL of 1 M NaCl
1 mol/L × 0.5 L × 58.44 g/mol = 29.22 g. Dissolve in about 400 mL of water, then make up to 500 mL.
250 mL of 1 M Tris base
Tris base has an MW of 121.14 g/mol. 1 × 0.25 × 121.14 = 30.29 g. Dissolve, adjust pH with HCl, and only then make up to volume, because acid addition changes the volume.
100 mL of 0.5 M EDTA, pH 8.0
Most labs buy disodium EDTA dihydrate (MW 372.24 g/mol). 0.5 × 0.1 × 372.24 = 18.61 g. It will not dissolve until the pH climbs towards 8.0, so add NaOH while stirring. People often think the powder is bad. It is just the pH.
| Target | Volume | MW (g/mol) | Weigh |
|---|---|---|---|
| 1 M NaCl | 500 mL | 58.44 | 29.22 g |
| 1 M Tris base | 250 mL | 121.14 | 30.29 g |
| 0.5 M Na₂EDTA·2H₂O | 100 mL | 372.24 | 18.61 g |
| 1 M MgCl₂·6H₂O | 100 mL | 203.30 | 20.33 g |
Hydrates, purity and the wrong MW
Read the label, not the protocol. MgCl₂ anhydrous is 95.21 g/mol and the hexahydrate is 203.30 g/mol. Use the wrong one and your stock is off by more than a factor of two. The same trap sits in CaCl₂, CuSO₄, sodium phosphates and EDTA salts.
For reagents with stated purity below about 98%, correct the mass: divide by the purity fraction. A 95% pure reagent needs 1/0.95, about 5% more mass, for the same molarity. For routine buffers this rarely matters. For standards and enzyme assays it does.
Bench habit: write the formula weight from the bottle on the tube label next to the molarity. Six months later, when a result looks odd, that one number tells you whether the stock was right.
Mistakes that cost a week
- Adding solute to the full final volume of water. The final volume ends up larger than planned, hence the concentration is lower.
- mL vs L. Typing 500 instead of 0.5 gives a 1000-fold error. Keep litres until the last step.
- Using the anhydrous MW for a hydrate. See above. Check every salt.
- pH-ing after making up to volume. Acid or base added later dilutes the solution a little and shifts the final concentration.
- Weighing hygroscopic salts slowly. CaCl₂ and MgCl₂ pull water from the air. Weigh fast and close the bottle.
If you would rather not do this by hand at 7 pm, BenchMate's molarity tool runs the same formula with the MW pulled from a verified compound list, so the hydrate question is answered for you.
Frequently asked questions
What is the formula for molarity?
Molarity equals moles of solute divided by litres of final solution. To find how much to weigh, use mass (g) = molarity (mol/L) × volume (L) × molecular weight (g/mol).
How do I make a 1 M solution?
Weigh the compound's molecular weight in grams for each litre you need, dissolve it in less than the final volume of water, then make up to the final volume. For 1 L of 1 M NaCl, weigh 58.44 g.
Does the hydrate form change the molarity calculation?
Yes. Use the molecular weight printed on your bottle, including water of hydration. MgCl₂·6H₂O is 203.30 g/mol while anhydrous MgCl₂ is 95.21 g/mol.
What is the difference between molarity and molality?
Molarity is moles per litre of solution. Molality is moles per kilogram of solvent. Bench protocols almost always use molarity.
22 core bench techniques, from molarity to sterilisation. Volume 1 of The Biotechnology Techniques Bible.