How It Works
This calculator works in 4 steps.
Enter the target
Type the molarity and volume, or the anhydrous mass a recipe calls for.
Enter the anhydrous MW
Type the MW of the water-free compound.
Enter the hydrate form
Type the number of water molecules, or type the hydrate MW from the label.
Read the hydrate mass
The calculator returns the mass of hydrate to weigh and the hydrate-to-anhydrous ratio.
The Formula
The formula adds 18.015 g/mol per water molecule to the anhydrous MW and scales the mass by the MW ratio.
MW_hydrate = MW_anhydrous + x × 18.015m_hydrate = m_anhydrous × MW_hydrate / MW_anhydrousm_hydrate = M × V × MW_hydrate- x:
- Waters of hydration per formula unit
- MW_hydrate / MW_anhydrous:
- Molecular weights of each form
- m:
- Mass
1 L of 1 M MgCl₂ needs 95.21 g anhydrous MgCl₂ or 203.30 g MgCl₂·6H₂O. The hydrate mass is 2.14 times the anhydrous mass.
Conversion Table
The conversion table compares anhydrous and hydrate masses for 100 mL of 1 M solution of 6 salts.
| Salt and hydrate | Anhydrous MW | Hydrate MW | Anhydrous mass | Hydrate mass |
|---|---|---|---|---|
| Copper(II) sulfate ·5H₂O | 159.6 g/mol | 249.7 g/mol | 15.96 g | 24.97 g |
| Magnesium chloride ·6H₂O | 95.21 g/mol | 203.3 g/mol | 9.521 g | 20.33 g |
| Calcium chloride ·2H₂O | 111 g/mol | 147 g/mol | 11.1 g | 14.7 g |
| Sodium phosphate dibasic ·7H₂O | 142 g/mol | 268.1 g/mol | 14.2 g | 26.81 g |
| Sodium carbonate ·10H₂O | 106 g/mol | 286.1 g/mol | 10.6 g | 28.61 g |
| Magnesium sulfate ·7H₂O | 120.4 g/mol | 246.5 g/mol | 12.04 g | 24.65 g |
What Is the Hydrated Reagent Calculator?
The Hydrated Reagent Calculator is a tool that returns the mass of a hydrated salt needed to deliver a target amount of the anhydrous compound, using the molecular weight of the hydrate form. The water of crystallization adds mass but no active solute.
Many salts come as hydrates, such as magnesium chloride hexahydrate (MgCl₂·6H₂O) and copper(II) sulfate pentahydrate (CuSO₄·5H₂O). A recipe written for the anhydrous form needs a larger weighed mass of the hydrate. Using the anhydrous MW with a hydrate gives a solution 47% weaker for MgCl₂·6H₂O. The calculator switches between forms and shows the mass ratio.
Common Uses
There are 5 main uses of this calculator.
Recipe substitution
Users follow anhydrous recipes with the hydrate on the shelf.
Culture media
Microbiologists weigh MgSO₄·7H₂O and CaCl₂·2H₂O for minimal media.
Phosphate buffers
Biochemists weigh Na₂HPO₄·7H₂O for PBS.
Inventory swaps
Labs switch between forms when one is out of stock.
Chemistry teaching
Instructors show how hydrate water changes formula mass.