The molar mass of copper(II) sulfate pentahydrate (CuSO4·5H2O) is 249.677 g/mol.
Case matters: CO is carbon monoxide, Co is
cobalt. Brackets and hydrates are fine — try
or
.
| Element | Atoms | Atomic mass | Mass contributed | Percent |
|---|---|---|---|---|
| Copper (Cu) | 1 | 63.546 | 63.546 | 25.45% |
| Hydrogen (H) | 10 | 1.0080 | 10.080 | 4.04% |
| Oxygen (O) | 9 | 15.999 | 143.991 | 57.67% |
| Sulfur (S) | 1 | 32.06 | 32.060 | 12.84% |
| Total | 21 | 249.677 | 100.00% |
63.546 + 10.080 + 143.991 + 32.060 = 249.677 g/mol
Grams and moles
25 g of copper(II) sulfate pentahydrate is 0.1001 mol, because 25 ÷ 249.677 = 0.1001.
These elements have no single atomic weight. Their isotopic mix varies slightly depending on where on Earth the sample came from, so the figure used here is the conventional value IUPAC publishes for when one number is needed: hydrogen is 1.0080, and really lies between 1.00784 and 1.00811; oxygen is 15.999, and really lies between 15.99903 and 15.99977; sulfur is 32.06, and really lies between 32.059 and 32.076.
The blue crystals on the shelf are the pentahydrate at 249.677 g/mol, not the anhydrous salt at 159.602. Five water molecules are built into the crystal structure, contributing 90.075 g/mol, which is 36% of the total mass.
Treat the blue crystals as if the water were not there and every result downstream is out by more than a third. This is the single most common weighing error in a school laboratory, and the bottle label is the only warning.
Heating drives the water off in stages and the colour goes with it, from blue through pale green to the white anhydrous powder. Add water and the blue returns immediately, which makes the compound a classic test for the presence of water. The colour belongs to the hydrated copper ion rather than to copper itself.
Oxygen accounts for 57.67% of the mass, copper 25.45%, sulfur 12.84% and hydrogen 4.04%. Most of what you are weighing out is water and oxygen, not metal.