🧂 Hydrated Salt Calculator
Calculate the molar mass, percent water of crystallization, and required weighed mass for a hydrated salt.
Frequently asked questions
Why does it matter whether I use the anhydrous or hydrated molar mass?
If a protocol calls for a specific mass of active anhydrous compound but you weigh out the hydrated salt using the anhydrous molar mass, you'll actually add less active compound than intended, since part of the hydrate's mass is water rather than the compound itself. Using the correct hydrated molar mass ensures you weigh out the right amount of hydrate to deliver the intended amount of anhydrous compound.
How do I know how many waters of crystallization my salt has?
This is specified in the compound's name or formula (e.g. 'pentahydrate' or '·5H2O' means n=5) and is usually printed on the reagent bottle label alongside the molecular formula and molar mass. If unsure, check the certificate of analysis or safety data sheet for your specific reagent lot.
Does %water of crystallization vary between different hydrate forms of the same salt?
Yes — many salts exist in multiple hydrate forms (e.g. anhydrous, monohydrate, dihydrate, pentahydrate), each with a different %water and molar mass. Always confirm which specific hydrate form your reagent bottle actually contains before calculating.
Accuracy & how this is derived
Derivation: Molar mass of the hydrate is calculated by adding n times the molar mass of water (18.015 g/mol) to the anhydrous salt's molar mass. Required hydrate mass is scaled from the target anhydrous mass by the ratio of hydrate to anhydrous molar mass.
Validated against: Basic stoichiometric calculation based on standard atomic/molecular weights — accuracy depends on correctly identifying the hydrate's water content (n) for the specific reagent used.
⚠️ For educational and research support only — verify critical results independently before use in regulated, clinical, or publication-bound work.
✔ Last updated: July 2026 · Report an error
Accounting for water of crystallization when weighing hydrated salts
Many common laboratory salts are supplied as hydrates, meaning each formula unit incorporates a fixed number of water molecules into its crystal structure, which adds real, weighable mass beyond the anhydrous compound alone. Calculating the hydrate's true molar mass by adding the mass contribution of its waters of crystallization, and using that value rather than the anhydrous molar mass when weighing out reagent, ensures that solution preparation and stoichiometric calculations accurately reflect the intended amount of active anhydrous compound.