The molar mass of magnesium sulfate heptahydrate (MgSO4·7H2O) is 246.466 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 |
|---|---|---|---|---|
| Hydrogen (H) | 14 | 1.0080 | 14.112 | 5.73% |
| Magnesium (Mg) | 1 | 24.305 | 24.305 | 9.86% |
| Oxygen (O) | 11 | 15.999 | 175.989 | 71.40% |
| Sulfur (S) | 1 | 32.06 | 32.060 | 13.01% |
| Total | 27 | 246.466 | 100.00% |
14.112 + 24.305 + 175.989 + 32.060 = 246.466 g/mol
Grams and moles
25 g of magnesium sulfate heptahydrate is 0.1014 mol, because 25 ÷ 246.466 = 0.1014.
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; magnesium is 24.305, and really lies between 24.304 and 24.307; 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.
Epsom salt is magnesium sulfate carrying seven waters of crystallisation, at 246.466 g/mol. Those seven waters contribute 126.105 g/mol, which is 51% of the total. More than half the mass of the crystals is water locked into the lattice.
Magnesium itself accounts for only 9.86%, so a 500 gram bag of Epsom salt contains under 50 grams of magnesium. Anyone calculating a dose from the weight of the salt rather than the weight of the metal will be out by a factor of ten.
You will see this compound quoted as 246.47 and also as 246.475. The higher figure comes from older atomic weights for sulfur and oxygen; the value here uses the current IUPAC table. With eleven oxygens in the formula, a revision in the fourth decimal place of oxygen moves the total by several thousandths.
The anhydrous form is 120.361 g/mol and is what you get after heating, which is also the basis of a standard gravimetric experiment for determining the number of waters.