Sodium phosphate monobasic monohydrate: molecular weight (formula weight)
Sodium phosphate monobasic monohydrate, H2NaO4P·H2O as weighed (Hill order), has a formula weight of 137.991 g/mol. 13.06 percent of that mass is water of crystallisation; the anhydrous formula, H2NaO4P, is 119.976 g/mol.
Weigh the form on your label, and if the label or certificate gives a formula weight, use that. A hydrate and its anhydrous form are not interchangeable: the same mass of each holds a different amount of substance.
Read the supplier's safety data sheet before you handle this reagent.
At a glance
| Formula as weighed (Hill order) | H2NaO4P·H2O |
|---|---|
| Water of crystallisation | One water molecule per formula unit |
| Formula weight as weighed | 137.991 g/mol |
| Formula weight, anhydrous | 119.976 g/mol |
| Water in the mass | 13.06 percent |
| PubChem | CID 516949; its weight, 137.99 g/mol, is a cross-check, not the source |
Molar mass is the SI name for this quantity; formula weight is the usual term for a salt or a hydrate, and labels and searches usually say molecular weight. On this page all three mean the mass of one mole of the formula as written, in grams per mole.
Atomic weights are the CIAAW standard values, with the conventional value for elements whose atomic weight varies in nature, such as hydrogen, carbon, sulfur, chlorine and lithium. The last decimal place is arithmetic, not measured precision: a real sample's formula weight can differ in the second decimal place, and by more for lithium salts.
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