To prepare a solution of magnesium sulfate you need the grams, and grams is molarity × volume in litres × 120.361 g/mol. A litre of 1 M takes 120.361 g; 500 mL of 0.1 M takes 6.0181 g; 250 mL of 0.01 M takes 300.9 mg. Enter the strength you want and the volume you want it in, and the calculator returns the mass to weigh together with the same figure in milligrams and as a percentage strength.
The figure comes from the formula. Magnesium sulfate is 24.305 (Mg) + 32.06 (S) + 4 × 15.999 (O), which is 120.361 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: 4 of the 6 atoms in a formula unit are oxygen, and they account for 63.996 g of the 120.361 g, or 53.17% by mass. The derivation table below breaks the whole molecule down element by element.
Order of operations matters more than precision here. Dissolve the solid in rather less than the final volume, then make up to the mark once it has all gone into solution and come back to room temperature — warm solutions read low when they cool. If the calculated mass is under about 20 mg, prepare ten times the strength and dilute it tenfold by pipette instead, because C₁V₁ = C₂V₂ carries less error than a marginal weighing.
The anhydrous salt, used as a laboratory drying agent. Epsom salt is the heptahydrate and has its own page — the two are not interchangeable by weight. At 120.361 g/mol it is heavier than 13 of the 34 salts covered here, and that ranking matters more than it looks: calcium chloride has a molar mass of 110.978 g/mol, so a gram of it contains 8.45% more formula units than a gram of magnesium sulfate. Weigh by mass and you are not weighing equal amounts of substance.
Purity is the usual gap between the calculation and the balance. A reagent sold at 98% means 2.4072 g of a nominal 120.361 g is something else, so for exact work you divide the weighed mass by the assay figure on the certificate. For most purposes the difference is smaller than the error in reading the meniscus, but it is systematic rather than random, so it does not average out.
The formula
c- The molarity you want, in mol/L
V- The volume you want to make, in millilitres
120.361- The molar mass of magnesium sulfate in g/mol, derived from its formula
m- The mass of magnesium sulfate to weigh out, in grams
How it works, step by step
- Enter the molarity you want, in mol/L.
- Enter the volume you want to make, in millilitres.
- The two are multiplied together and by 120.361 g/mol.
- The result is the mass to weigh; dissolve it in part of the volume and then make up to the mark.
Worked examples
250 mL of 0.1 M magnesium sulfate
Weigh 3.00903 g. That is 0.1 M × 0.25 L × 120.361 g/mol, and it comes to 3009.03 mg if your balance is set to milligrams. Dissolve in about 175 mL first, then make up to 250 mL.
500 mL of 0.5 M magnesium sulfate
Weigh 30.0903 g. That is 0.5 M × 0.5 L × 120.361 g/mol, and it comes to 30,090.2 mg if your balance is set to milligrams. Dissolve in about 350 mL first, then make up to 500 mL.
1000 mL of 1 M magnesium sulfate
Weigh 120.361 g. That is 1 M × 1 L × 120.361 g/mol, and it comes to 120,361 mg if your balance is set to milligrams. Dissolve in about 700 mL first, then make up to 1000 mL.
How to read your score
Frequently asked questions
What is the molar mass of magnesium sulfate (MgSO4)?
It is 120.361 g/mol. That is the sum of the standard atomic weights of every atom in the formula: 24.305 (Mg) + 32.06 (S) + 4 × 15.999 (O). One mole of magnesium sulfate therefore weighs 120.361 g, and a gram of it is 8.3083 mmol.
What percentage of magnesium sulfate is oxygen?
53.17% by mass. Each formula unit contains 4 oxygen atoms contributing 63.996 g of the 120.361 g total, so 100 g of magnesium sulfate contains 53.17 g of oxygen and a kilogram contains 531.7 g of it.
How much magnesium sulfate do I need for 1 litre of 1 M solution?
120.361 g — the molar mass in grams, which is what a 1 molar solution means. For 1 L of 0.1 M it is 12.0361 g, and for 1 L of 0.01 M it is 1.20361 g.
How much for 100 mL of 0.1 M?
1.20361 g. The volume is a tenth of a litre and the strength a tenth of molar, so the mass is a hundredth of 120.361 g. In milligrams that is 1203.61 mg.
Do I dissolve the solid first or make up the volume first?
Dissolve first, in perhaps 70% of the final volume, then make up to the mark. Adding solid to a full flask overshoots the volume and leaves the solution weak, and undissolved solid at the mark means the strength keeps changing as it goes in.
Can I dilute a stock solution instead of weighing?
Yes, and it is more accurate for small amounts. C₁V₁ = C₂V₂: to get 500 mL of 0.01 M from a 0.5 M stock, take 10 mL of stock and make up to 500 mL. Weighing the 601.81 mg that 500 mL of 0.01 M needs directly is the harder of the two.
Solution recipes for magnesium sulfate
| Element | Atoms | Atomic weight | Contribution (g/mol) | By mass |
|---|---|---|---|---|
| Oxygen (O) | 4 | 15.999 | 63.996 | 53.17% |
| Sulfur (S) | 1 | 32.06 | 32.06 | 26.64% |
| Magnesium (Mg) | 1 | 24.305 | 24.305 | 20.19% |
| Total — one mole of magnesium sulfate | 120.361 | 100% |
Standard atomic weights, IUPAC 2021. The contribution column is atoms × atomic weight, and the total is the molar mass this page uses: 120.361 g/mol.
| Target | for 100 mL | for 250 mL | for 500 mL | for 1 L |
|---|---|---|---|---|
| 0.001 M | 12.0361 mg | 30.0903 mg | 60.1805 mg | 120.361 mg |
| 0.005 M | 60.1805 mg | 150.451 mg | 300.903 mg | 601.805 mg |
| 0.01 M | 120.361 mg | 300.903 mg | 601.805 mg | 1.20361 g |
| 0.05 M | 601.805 mg | 1.50451 g | 3.00903 g | 6.01805 g |
| 0.1 M | 1.20361 g | 3.00903 g | 6.01805 g | 12.0361 g |
| 0.15 M | 1.80541 g | 4.51354 g | 9.02707 g | 18.0541 g |
| 0.2 M | 2.40722 g | 6.01805 g | 12.0361 g | 24.0722 g |
| 0.25 M | 3.00903 g | 7.52256 g | 15.0451 g | 30.0903 g |
| 0.5 M | 6.01805 g | 15.0451 g | 30.0903 g | 60.1805 g |
| 1 M | 12.0361 g | 30.0903 g | 60.1805 g | 120.361 g |
| 2 M | 24.0722 g | 60.1805 g | 120.361 g | 240.722 g |
Each cell is molarity × volume in litres × 120.361 g/mol. Dissolve the solid first and then make up to the marked volume — adding solid to a full volume overshoots it.
| Compound | Formula | Molar mass (g/mol) | Millimoles in 1 g |
|---|---|---|---|
| Potassium nitrate | KNO3 | 101.1023 | 9.891 |
| Sodium carbonate | Na2CO3 | 105.9875 | 9.4351 |
| Calcium chloride | CaCl2 | 110.978 | 9.0108 |
| Magnesium sulfate (this page) | MgSO4 | 120.361 | 8.3083 |
| Ammonium sulfate | (NH4)2SO4 | 132.134 | 7.5681 |
| Monopotassium phosphate | KH2PO4 | 136.0841 | 7.3484 |
| Disodium phosphate | Na2HPO4 | 141.9573 | 7.0444 |
Ordered by molar mass. The last column is 1000/M, which is the number a weighed gram actually gives you.