Percentage strength for Epsom salt is read as % w/v: grams of solute in 100 mL of solution. So 1% is 1 g per 100 mL, which for this compound works out at 0.040574 mol/L, and 0.9% is 9 g per litre or 0.036516 mol/L. The calculator takes a percentage and a volume and returns the grams to weigh, with the molarity and mg/mL equivalents beside it.
The figure comes from the formula. Epsom salt is 24.305 (Mg) + 32.06 (S) + 11 × 15.999 (O) + 14 × 1.008 (H), which is 246.466 g/mol, taking standard atomic weights, and every page here computes that sum rather than quoting it. Oxygen makes up the largest share of the mass: 11 of the 27 atoms in a formula unit are oxygen, and they account for 175.989 g of the 246.466 g, or 71.4% by mass. The derivation table below breaks the whole molecule down element by element.
Three different percentages are in circulation and they are not interchangeable. Per cent w/v is mass in a volume, the one used here and on medical labels; per cent w/w is mass in mass, the one on drums of concentrated reagent; per cent v/v is volume in volume, used for liquids mixed into liquids. For a dilute aqueous solution w/v and w/w are within a percent of each other, but for anything concentrated the density has to come into it.
Magnesium sulfate heptahydrate: bath salts, a magnesium source for plants, and a laboratory standard. Seven waters make it twice the weight per mole. At 246.466 g/mol it is heavier than 26 of the 34 salts covered here, and that ranking matters more than it looks: potassium sulfate has a molar mass of 174.2526 g/mol, so a gram of it contains 41.4% more formula units than a gram of Epsom salt. Weigh by mass and you are not weighing equal amounts of substance.
Watch which form you have. This page is the hydrate, MgSO4·7H2O, at 246.466 g/mol; the anhydrous salt is MgSO4 at 120.361 g/mol. The 126.105 g of water per mole is 51.17% of the weight, so weighing the hydrate as though it were anhydrous puts you 51.17% short on the compound itself. Suppliers sell both, and the label is the only way to tell them apart by eye.
The formula
p- The strength you want, as % w/v
V- The volume of solution, in millilitres
m- The mass of Epsom salt to weigh out, in grams
246.466- The molar mass of Epsom salt in g/mol, derived from its formula
How it works, step by step
- Enter the percentage strength you want, as % w/v.
- Enter the volume of solution in millilitres.
- Grams needed is percentage × volume ÷ 100, since 1% w/v is 1 g per 100 mL.
- The molarity that strength corresponds to for Epsom salt is shown beneath, using 246.466 g/mol.
Worked examples
0.9% w/v Epsom salt in 500 mL
0.9% w/v is 0.9 g per 100 mL, so 500 mL needs 4.5 g. In molar terms that solution is 0.036516 mol/L, and it is 9 mg/mL if you are dosing by volume.
5% w/v Epsom salt in 100 mL
5% w/v is 5 g per 100 mL, so 100 mL needs 5 g. In molar terms that solution is 0.20287 mol/L, and it is 50 mg/mL if you are dosing by volume.
10% w/v Epsom salt in 1000 mL
10% w/v is 10 g per 100 mL, so 1000 mL needs 100 g. In molar terms that solution is 0.40574 mol/L, and it is 100 mg/mL if you are dosing by volume.
How to read your score
Frequently asked questions
What is the molar mass of Epsom salt (MgSO4·7H2O)?
It is 246.466 g/mol. That is the sum of the standard atomic weights of every atom in the formula: 24.305 (Mg) + 32.06 (S) + 11 × 15.999 (O) + 14 × 1.008 (H). One mole of Epsom salt therefore weighs 246.466 g, and a gram of it is 4.0574 mmol.
What percentage of Epsom salt is oxygen?
71.4% by mass. Each formula unit contains 11 oxygen atoms contributing 175.989 g of the 246.466 g total, so 100 g of Epsom salt contains 71.4 g of oxygen and a kilogram contains 714.05 g of it.
How many grams of Epsom salt make a 1% w/v solution?
1 g in 100 mL of solution, 5 g in 500 mL, 10 g in a litre. The percentage is fixed to the volume, not to the compound, so those masses are the same whatever is being dissolved — what changes with the compound is the molarity they come to.
What molarity is a 1% w/v solution of Epsom salt?
0.0405735 mol/L, because 1% w/v is 10 g/L and 246.466 g/L is one molar. A 0.9% solution is 0.036516 mol/L and a 5% solution is 0.20287 mol/L.
Is % w/v the same as % w/w?
No. Per cent w/v is grams per 100 mL of solution; per cent w/w is grams per 100 g of solution. They coincide only when the solution has a density of 1.00 g/mL, which dilute aqueous solutions approximately do and concentrated ones do not. Labels on concentrated reagents are usually w/w.
What is 1% w/v in mg/mL?
10 mg/mL, since 1 g in 100 mL is 1000 mg in 100 mL. For Epsom salt that is also 40.574 mmol/L, which is the form a dose calculation usually wants.
Percentage strengths of Epsom salt
| Element | Atoms | Atomic weight | Contribution (g/mol) | By mass |
|---|---|---|---|---|
| Oxygen (O) | 11 | 15.999 | 175.989 | 71.4% |
| Sulfur (S) | 1 | 32.06 | 32.06 | 13.01% |
| Magnesium (Mg) | 1 | 24.305 | 24.305 | 9.861% |
| Hydrogen (H) | 14 | 1.008 | 14.112 | 5.726% |
| Total — one mole of Epsom salt | 246.466 | 100% |
Standard atomic weights, IUPAC 2021. The contribution column is atoms × atomic weight, and the total is the molar mass this page uses: 246.466 g/mol.
| Strength | per 100 mL | per 500 mL | per 1 L | Molarity (mol/L) |
|---|---|---|---|---|
| 0.1% w/v | 100 mg | 500 mg | 1 g | 0.0040574 |
| 0.25% w/v | 250 mg | 1.25 g | 2.5 g | 0.010143 |
| 0.5% w/v | 500 mg | 2.5 g | 5 g | 0.020287 |
| 0.9% w/v | 900 mg | 4.5 g | 9 g | 0.036516 |
| 1% w/v | 1 g | 5 g | 10 g | 0.040574 |
| 2% w/v | 2 g | 10 g | 20 g | 0.081147 |
| 3% w/v | 3 g | 15 g | 30 g | 0.12172 |
| 5% w/v | 5 g | 25 g | 50 g | 0.20287 |
| 10% w/v | 10 g | 50 g | 100 g | 0.40574 |
| 20% w/v | 20 g | 100 g | 200 g | 0.81147 |
A 1% w/v solution is 1 g in 100 mL, which for this compound is 0.040574 mol/L because 10 g/L divided by 246.466 g/mol gives that figure.
| Compound | Formula | Molar mass (g/mol) | Millimoles in 1 g |
|---|---|---|---|
| Silver nitrate | AgNO3 | 169.8722 | 5.8868 |
| Gypsum | CaSO4·2H2O | 172.164 | 5.8084 |
| Potassium sulfate | K2SO4 | 174.2526 | 5.7388 |
| Epsom salt (this page) | MgSO4·7H2O | 246.466 | 4.0574 |
| Sodium thiosulfate pentahydrate | Na2S2O3·5H2O | 248.1715 | 4.0295 |
| Copper(II) sulfate pentahydrate | CuSO4·5H2O | 249.677 | 4.0052 |
| Sodium citrate | Na3C6H5O7 | 258.0683 | 3.8749 |
Ordered by molar mass. The last column is 1000/M, which is the number a weighed gram actually gives you.