Going from grams to moles for potassium sulfate means dividing by its molar mass, 174.2526 g/mol. One gram is 0.0057388 mol, or 5.7388 mmol; 100 mg is 0.57388 mmol; and a 174.2526 g portion is exactly one mole. The division is the whole calculation, but the molar mass has to be the one for the exact formula you have, K2SO4, which is where these figures come from.
The figure comes from the formula. Potassium sulfate is 2 × 39.0983 (K) + 32.06 (S) + 4 × 15.999 (O), which is 174.2526 g/mol, taking standard atomic weights, and every page here computes that sum rather than quoting it. Potassium makes up the largest share of the mass: 2 of the 7 atoms in a formula unit are potassium, and they account for 78.1966 g of the 174.2526 g, or 44.88% by mass. The derivation table below breaks the whole molecule down element by element.
Balance resolution is the practical limit here. On a two-decimal balance the smallest step is 10 mg, which is 0.05739 mmol of potassium sulfate; on a three-decimal balance it is 1 mg, or 5.739 µmol. To hit a target within 1% you therefore need to weigh at least a gram on the first and at least 100 mg on the second, and below that a stock solution measured by pipette beats weighing.
A chloride-free potassium fertiliser, preferred where salt sensitivity matters. It is much less soluble than the chloride, about 120 g per litre. At 174.2526 g/mol it is heavier than 25 of the 34 salts covered here, and that ranking matters more than it looks: gypsum has a molar mass of 172.164 g/mol, so a gram of it contains 1.21% more formula units than a gram of potassium 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, 3.4851 g of every 174.2526 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 you weighed, in grams
174.2526- The molar mass of potassium sulfate in g/mol, derived from its formula
n- The amount of substance in moles
How it works, step by step
- Enter the mass you weighed, in grams.
- It is divided by 174.2526 g/mol, the molar mass of potassium sulfate.
- The result is the amount of substance in moles, with millimoles and micromoles beneath it.
- Check it against the table if you want the figure for a standard weighing rather than a typed mass.
Worked examples
0.25 g of potassium sulfate in moles
0.25 ÷ 174.2526 = 0.0014347 mol, or 1.4347 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 5.739 µmol, so quoting the answer to more than four significant figures claims a precision the weighing did not have.
2 g of potassium sulfate in moles
2 ÷ 174.2526 = 0.0114776 mol, or 11.478 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 5.739 µmol, so quoting the answer to more than four significant figures claims a precision the weighing did not have.
25 g of potassium sulfate in moles
25 ÷ 174.2526 = 0.14347 mol, or 143.47 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 5.739 µmol, so quoting the answer to more than four significant figures claims a precision the weighing did not have.
How to read your score
Frequently asked questions
What is the molar mass of potassium sulfate (K2SO4)?
It is 174.2526 g/mol. That is the sum of the standard atomic weights of every atom in the formula: 2 × 39.0983 (K) + 32.06 (S) + 4 × 15.999 (O). One mole of potassium sulfate therefore weighs 174.2526 g, and a gram of it is 5.7388 mmol.
What percentage of potassium sulfate is potassium?
44.88% by mass. Each formula unit contains 2 potassium atoms contributing 78.1966 g of the 174.2526 g total, so 100 g of potassium sulfate contains 44.88 g of potassium and a kilogram contains 448.75 g of it.
How do I convert grams of potassium sulfate to moles?
Divide the mass in grams by 174.2526 g/mol. So 1 g is 0.0057388 mol, 10 g is 0.057388 mol and 100 g is 0.57388 mol. Nothing else enters the calculation — no volume, no temperature, no concentration.
How many moles is 100 g of potassium sulfate?
0.57388 mol, which is 573.88 mmol. Read the other way, one mole of potassium sulfate is 174.2526 g, so 100 g is 0.5739 of a mole.
What about milligrams and micromoles?
The same division, scaled. A milligram of potassium sulfate is 5.7388 µmol, 10 mg is 57.388 µmol and 100 mg is 0.57388 mmol. Working in µmol per mg avoids the string of leading zeros that mol per g produces at this scale.
Does purity change the number of moles?
Yes, in proportion. A 98% pure sample weighing 1 g contains 0.98 g of potassium sulfate, which is 0.00562402 mol rather than 0.0057388 mol. The calculator assumes the mass you enter is the compound itself, so divide by the assay figure first if you need the corrected amount.
Grams to moles reference for potassium sulfate
| Element | Atoms | Atomic weight | Contribution (g/mol) | By mass |
|---|---|---|---|---|
| Potassium (K) | 2 | 39.0983 | 78.1966 | 44.88% |
| Oxygen (O) | 4 | 15.999 | 63.996 | 36.73% |
| Sulfur (S) | 1 | 32.06 | 32.06 | 18.4% |
| Total — one mole of potassium sulfate | 174.2526 | 100% |
Standard atomic weights, IUPAC 2021. The contribution column is atoms × atomic weight, and the total is the molar mass this page uses: 174.2526 g/mol.
| On the balance | Moles | Millimoles | Micromoles |
|---|---|---|---|
| 1 mg | 0.000006 | 0.0057388 | 5.7388 |
| 5 mg | 0.000029 | 0.028694 | 28.694 |
| 10 mg | 0.000057 | 0.057388 | 57.388 |
| 25 mg | 0.00014347 | 0.14347 | 143.47 |
| 50 mg | 0.00028694 | 0.28694 | 286.94 |
| 100 mg | 0.00057388 | 0.57388 | 573.88 |
| 250 mg | 0.0014347 | 1.4347 | 1434.7 |
| 500 mg | 0.0028694 | 2.8694 | 2869.4 |
| 1 g | 0.0057388 | 5.7388 | 5738.8 |
| 2 g | 0.0114776 | 11.4776 | 11,477.6 |
| 5 g | 0.028694 | 28.694 | 28,694 |
| 10 g | 0.057388 | 57.388 | 57,388 |
| 25 g | 0.14347 | 143.47 | 143,470 |
| 50 g | 0.28694 | 286.94 | 286,940 |
| 100 g | 0.57388 | 573.88 | 573,880 |
| 250 g | 1.4347 | 1434.7 | 1434698.822284 |
| 500 g | 2.8694 | 2869.4 | 2869397.644569 |
Every row is the mass divided by 174.2526 g/mol. A balance reading to 1 mg resolves 5.739 µmol of this compound, which is the smallest step the middle columns can really move in.
| Compound | Formula | Molar mass (g/mol) | Millimoles in 1 g |
|---|---|---|---|
| Potassium iodide | KI | 166.0028 | 6.024 |
| Silver nitrate | AgNO3 | 169.8722 | 5.8868 |
| Gypsum | CaSO4·2H2O | 172.164 | 5.8084 |
| Potassium sulfate (this page) | K2SO4 | 174.2526 | 5.7388 |
| Epsom salt | 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 |
Ordered by molar mass. The last column is 1000/M, which is the number a weighed gram actually gives you.