Molarity to Mass Calculator - How Much Reagent to Weigh
How many grams to weigh out for a target molarity and volume, with the hydrate trap caught.
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Mass equals concentration times volume times formula weight, which is easy until the formula weight is the wrong one. EDTA is the reagent that makes the point: the disodium dihydrate sitting in most labs is 372.24 g/mol and the free acid is 292.24, so reaching for the wrong entry puts your buffer 27% out - silently, because the solution still looks exactly like a solution. Every reagent here is therefore named with its hydration state, and a name that matches two forms is refused with both of them named rather than resolved to a guess. Formula weights are computed from the molecular formula rather than transcribed, so each one can be checked against the bottle in front of you.
Hydration state is part of the name: the disodium dihydrate of EDTA and the free acid differ by 27% in formula weight.
EDTA disodium dihydrate — formula weight computed from C10H14N2Na2O8·2H2O. Dissolve and make up to 500 mL.
- Using EDTA disodium dihydrate (372.24 g/mol). A different hydration state of the same reagent has a different formula weight, so check the bottle matches.
How to use the Solution Prep Calculator tool
- 1Enter the concentration you want and its unit.
- 2Enter the volume you are making and its unit.
- 3Pick the reagent - matching the hydration state printed on your bottle - or choose the last option and type the formula weight yourself.
- 4Weigh out the mass shown, dissolve, and make up to the final volume.
Frequently asked questions
How do I calculate how many grams to weigh for a given molarity?
Mass in grams equals the concentration in mol/L multiplied by the volume in litres multiplied by the formula weight in g/mol. For 500 mL of 0.5 M EDTA disodium dihydrate, that is 0.5 x 0.5 x 372.24 = 93.06 g. The tool handles the unit conversions and picks the formula weight from the reagent you name.
Why does the hydration state of a reagent matter?
Because the water of crystallisation counts towards the mass you weigh but not towards the moles of the active compound. EDTA disodium dihydrate is 372.24 g/mol against 292.24 for the free acid - weigh by the wrong one and your buffer is 27% off. The same applies to magnesium chloride hexahydrate against anhydrous, and to sodium acetate trihydrate against anhydrous.
What if my reagent is not in the list?
Choose the last option in the reagent picker and type the formula weight from the bottle. Every supplier prints it on the label, and it is the only number this calculation needs beyond your concentration and volume.
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