Molarity is moles of solute per litre of solution, so for aluminium sulfate it is the mass divided by 342.13108 g/mol and then by the volume in litres. One gram in 100 mL is 0.029229 mol/L; the same gram in a litre is 0.0029229 mol/L; and 342.1311 g in a litre is a 1 M solution. Enter a mass and a volume and the calculator does both divisions, and also reports the strength as g/L and % w/v.
The figure comes from the formula. Aluminium sulfate is 2 × 26.9815 (Al) + 3 × 32.06 (S) + 12 × 15.999 (O), which is 342.13108 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: 12 of the 17 atoms in a formula unit are oxygen, and they account for 191.988 g of the 342.1311 g, or 56.12% by mass. The derivation table below breaks the whole molecule down element by element.
The volume in the denominator is the volume of the finished solution, not the volume of water you started with. Dissolving 342.131 g of a solid adds a little volume of its own, so a solution made up to the mark in a volumetric flask is at the molarity you calculated while the same solid stirred into a litre of water is slightly weaker. At dilute strengths the difference is negligible; at 1 M and above it is not.
The flocculant in water treatment and the mordant in dyeing. It hydrolyses in water to give an acidic solution, which is what makes it lower soil pH. At 342.1311 g/mol it is the heaviest of the 34 salts covered here, and that ranking matters more than it looks: potassium dichromate has a molar mass of 294.1818 g/mol, so a gram of it contains 16.3% more formula units than a gram of aluminium sulfate. Weigh by mass and you are not weighing equal amounts of substance.
Two practical things move the result. Purity below 100% means the weighed mass overstates how much compound you have — at 98% assay, 6.8426 g of every 342.1311 g is not the compound — and any water picked up from the air does the same. Both are systematic, so they shift every solution made from that jar in the same direction.
The formula
m- The mass of aluminium sulfate dissolved, in grams
V- The final volume of the solution, in millilitres
342.13108- The molar mass of aluminium sulfate in g/mol, derived from its formula
c- The concentration in moles per litre
How it works, step by step
- Enter the mass of aluminium sulfate you dissolved, in grams.
- Enter the final volume of the solution in millilitres.
- The mass is divided by 342.13108 g/mol and then by the volume in litres.
- The result is molarity in mol/L, with mmol/L, g/L and % w/v given alongside.
Worked examples
1 g of aluminium sulfate made up to 100 mL
1 g is 0.00292286 mol. Divided by 0.1 L that is 0.0292286 mol/L, or 29.229 mmol/L. The same solution is 10 g/L and 1% w/v — four ways of writing one strength.
5 g of aluminium sulfate made up to 500 mL
5 g is 0.0146143 mol. Divided by 0.5 L that is 0.0292286 mol/L, or 29.229 mmol/L. The same solution is 10 g/L and 1% w/v — four ways of writing one strength.
25 g of aluminium sulfate made up to 1000 mL
25 g is 0.0730714 mol. Divided by 1 L that is 0.0730714 mol/L, or 73.071 mmol/L. The same solution is 25 g/L and 2.5% w/v — four ways of writing one strength.
How to read your score
Frequently asked questions
What is the molar mass of aluminium sulfate (Al2(SO4)3)?
It is 342.13108 g/mol. That is the sum of the standard atomic weights of every atom in the formula: 2 × 26.9815 (Al) + 3 × 32.06 (S) + 12 × 15.999 (O). One mole of aluminium sulfate therefore weighs 342.1311 g, and a gram of it is 2.9229 mmol.
What percentage of aluminium sulfate is oxygen?
56.12% by mass. Each formula unit contains 12 oxygen atoms contributing 191.988 g of the 342.1311 g total, so 100 g of aluminium sulfate contains 56.12 g of oxygen and a kilogram contains 561.15 g of it.
How do I calculate the molarity of a aluminium sulfate solution?
Divide the mass in grams by 342.13108 g/mol to get moles, then divide by the volume of solution in litres. Both divisions are in the calculator: enter grams and millilitres and it reports mol/L.
What is the molarity of 1 g of aluminium sulfate in 100 mL?
0.0292286 mol/L, or 29.229 mmol/L. The same gram in 250 mL gives 0.0116914 mol/L and in a litre 0.00292286 mol/L, since molarity falls in inverse proportion to the volume.
Is molarity per litre of water or per litre of solution?
Per litre of finished solution. That is why volumetric glassware is filled to a mark after the solid has dissolved rather than measured out first. Molality, which is per kilogram of solvent, is the quantity defined the other way and is not what this page computes.
How do mol/L, g/L and % w/v relate for aluminium sulfate?
One mol/L of aluminium sulfate is 342.1311 g/L, which is 34.213% w/v. Going the other way, 1% w/v is 10 g/L and therefore 0.029229 mol/L. All three describe the same solution.
Molarity of aluminium sulfate by mass and volume
| Element | Atoms | Atomic weight | Contribution (g/mol) | By mass |
|---|---|---|---|---|
| Oxygen (O) | 12 | 15.999 | 191.988 | 56.12% |
| Sulfur (S) | 3 | 32.06 | 96.18 | 28.11% |
| Aluminium (Al) | 2 | 26.98154 | 53.96308 | 15.77% |
| Total — one mole of aluminium sulfate | 342.13108 | 100% |
Standard atomic weights, IUPAC 2021. The contribution column is atoms × atomic weight, and the total is the molar mass this page uses: 342.13108 g/mol.
| Solute | in 100 mL | in 250 mL | in 500 mL | in 1 L |
|---|---|---|---|---|
| 10 mg | 0.00029229 | 0.00011691 | 0.000058 | 0.000029 |
| 50 mg | 0.0014614 | 0.00058457 | 0.00029229 | 0.00014614 |
| 100 mg | 0.0029229 | 0.0011691 | 0.00058457 | 0.00029229 |
| 250 mg | 0.0073071 | 0.0029229 | 0.0014614 | 0.00073071 |
| 500 mg | 0.014614 | 0.0058457 | 0.0029229 | 0.0014614 |
| 1 g | 0.029229 | 0.011691 | 0.0058457 | 0.0029229 |
| 2 g | 0.058457 | 0.023383 | 0.011691 | 0.0058457 |
| 5 g | 0.14614 | 0.058457 | 0.029229 | 0.014614 |
| 10 g | 0.29229 | 0.11691 | 0.058457 | 0.029229 |
| 25 g | 0.73071 | 0.29229 | 0.14614 | 0.073071 |
| 50 g | 1.4614 | 0.58457 | 0.29229 | 0.14614 |
| 100 g | 2.9229 | 1.1691 | 0.58457 | 0.29229 |
All figures in mol/L. Each is the mass divided by 342.13108 g/mol and then by the volume in litres, so the columns differ only by that volume.
| Compound | Formula | Molar mass (g/mol) | Millimoles in 1 g |
|---|---|---|---|
| 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 |
| Iron(II) sulfate heptahydrate | FeSO4·7H2O | 278.006 | 3.597 |
| Zinc sulfate heptahydrate | ZnSO4·7H2O | 287.541 | 3.4778 |
| Potassium dichromate | K2Cr2O7 | 294.1818 | 3.3993 |
| Aluminium sulfate (this page) | Al2(SO4)3 | 342.1311 | 2.9229 |
Ordered by molar mass. The last column is 1000/M, which is the number a weighed gram actually gives you.