Molarity calculator
The mass to weigh for a molar solution is molarity times volume times formula weight: grams = M × V(L) × FW(g/mol). One litre of 1 M NaCl (formula weight 58.44 g/mol) needs 58.44 g; 50 ml of 100 mM Tris (FW 121.14) needs 0.61 g.
Result
mass (g) = molarity (mol/L) × volume (L) × formula weight (g/mol). Default example: 1 L of 1 M NaCl (FW 58.44) needs 58.44 g.
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Formula
mass (g) = molarity (mol/L) × volume (L) × formula weight (g/mol)
Source: Definition of molarity; Promega BioMath molarity calculator
Worked example
Given: 1 L of 1 M NaCl, formula weight 58.44 g/mol
- 1.mass = 1 mol/L × 1 L × 58.44 g/mol = 58.44 g.
Weigh 58.44 g of NaCl and bring to 1 L
How the calculation flows
Units & constants
| Inputs | Formula weight (g/mol), molarity (M, mM or µM), volume (L, ml or µl) |
|---|---|
| Output | Mass in g and mg |
| FW source | Reagent bottle label or supplier datasheet — use the hydrate's FW if using a hydrate |
| Common FWs | NaCl 58.44 · Tris base 121.14 · EDTA·2H₂O·2Na 372.24 · glucose 180.16 |
| Price | Free |
How do you make a molar solution correctly?
Weigh the calculated mass, dissolve in less than the final volume, adjust pH if the recipe calls for it, then bring to the final volume. Adding the compound to a pre-measured full volume overshoots, because the solute itself adds volume — the classic reason a '1 M' stock titrates low.
Which formula weight should you use for hydrates?
The one on the bottle you are actually holding. MgCl₂ anhydrous is 95.21 g/mol but the usual hexahydrate is 203.30 — the water of crystallisation is part of what you weigh. Using the anhydrous FW with a hydrate makes the solution more than twice as dilute as intended in that case.
Molarity, molality or percent — which is this?
Molarity: moles per litre of final solution, the standard for bench buffers. Molality (moles per kg of solvent) appears in physical chemistry, and percent solutions (w/v) in gel and media recipes. Converting percent to molarity is dividing (g per 100 ml × 10) by the formula weight.
Frequently asked questions
- How many grams of NaCl for 500 ml of 5 M?
- mass = 5 × 0.5 × 58.44 = 146.1 g. Note that 5 M NaCl is near saturation (~6.1 M at room temperature) — dissolve with stirring before bringing to volume.
- What if my target is in mM?
- Convert to molar by dividing by 1,000 (the calculator's unit selector does this). 100 mM in 50 ml of Tris: 0.1 × 0.05 × 121.14 = 0.606 g.
- Where do I find the formula weight?
- On the reagent bottle ('FW' or 'MW'), the supplier's catalogue page, or the certificate of analysis. For hydrates, use the hydrate's FW printed on that lot's label.
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