Going from grams to moles for monopotassium phosphate means dividing by its molar mass, 136.08406 g/mol. One gram is 0.0073484 mol, or 7.3484 mmol; 100 mg is 0.73484 mmol; and a 136.0841 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, KH2PO4, which is where these figures come from.
The figure comes from the formula. Monopotassium phosphate is 39.0983 (K) + 2 × 1.008 (H) + 30.9738 (P) + 4 × 15.999 (O), which is 136.08406 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 8 atoms in a formula unit are oxygen, and they account for 63.996 g of the 136.0841 g, or 47.03% 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 0.07348 mmol of monopotassium phosphate; on a three-decimal balance it is 1 mg, or 7.348 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.
The acidic half of a phosphate buffer and a hydroponic nutrient. Paired with the disodium salt it holds pH near 7, which is why both appear in recipes. At 136.0841 g/mol it is heavier than 15 of the 34 salts covered here, and that ranking matters more than it looks: ammonium sulfate has a molar mass of 132.134 g/mol, so a gram of it contains 2.99% more formula units than a gram of monopotassium phosphate. 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, 2.7217 g of every 136.0841 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
136.08406- The molar mass of monopotassium phosphate 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 136.08406 g/mol, the molar mass of monopotassium phosphate.
- 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 monopotassium phosphate in moles
0.25 ÷ 136.0841 = 0.0018371 mol, or 1.8371 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 7.348 µmol, so quoting the answer to more than four significant figures claims a precision the weighing did not have.
2 g of monopotassium phosphate in moles
2 ÷ 136.0841 = 0.0146968 mol, or 14.697 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 7.348 µmol, so quoting the answer to more than four significant figures claims a precision the weighing did not have.
25 g of monopotassium phosphate in moles
25 ÷ 136.0841 = 0.18371 mol, or 183.71 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 7.348 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 monopotassium phosphate (KH2PO4)?
It is 136.08406 g/mol. That is the sum of the standard atomic weights of every atom in the formula: 39.0983 (K) + 2 × 1.008 (H) + 30.9738 (P) + 4 × 15.999 (O). One mole of monopotassium phosphate therefore weighs 136.0841 g, and a gram of it is 7.3484 mmol.
What percentage of monopotassium phosphate is oxygen?
47.03% by mass. Each formula unit contains 4 oxygen atoms contributing 63.996 g of the 136.0841 g total, so 100 g of monopotassium phosphate contains 47.03 g of oxygen and a kilogram contains 470.27 g of it.
How do I convert grams of monopotassium phosphate to moles?
Divide the mass in grams by 136.08406 g/mol. So 1 g is 0.0073484 mol, 10 g is 0.073484 mol and 100 g is 0.73484 mol. Nothing else enters the calculation — no volume, no temperature, no concentration.
How many moles is 100 g of monopotassium phosphate?
0.73484 mol, which is 734.84 mmol. Read the other way, one mole of monopotassium phosphate is 136.0841 g, so 100 g is 0.7348 of a mole.
What about milligrams and micromoles?
The same division, scaled. A milligram of monopotassium phosphate is 7.3484 µmol, 10 mg is 73.484 µmol and 100 mg is 0.73484 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 monopotassium phosphate, which is 0.00720143 mol rather than 0.0073484 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 monopotassium phosphate
| Element | Atoms | Atomic weight | Contribution (g/mol) | By mass |
|---|---|---|---|---|
| Oxygen (O) | 4 | 15.999 | 63.996 | 47.03% |
| Potassium (K) | 1 | 39.0983 | 39.0983 | 28.73% |
| Phosphorus (P) | 1 | 30.97376 | 30.97376 | 22.76% |
| Hydrogen (H) | 2 | 1.008 | 2.016 | 1.481% |
| Total — one mole of monopotassium phosphate | 136.08406 | 100% |
Standard atomic weights, IUPAC 2021. The contribution column is atoms × atomic weight, and the total is the molar mass this page uses: 136.08406 g/mol.
| On the balance | Moles | Millimoles | Micromoles |
|---|---|---|---|
| 1 mg | 0.000007 | 0.0073484 | 7.3484 |
| 5 mg | 0.000037 | 0.036742 | 36.742 |
| 10 mg | 0.000073 | 0.073484 | 73.484 |
| 25 mg | 0.00018371 | 0.18371 | 183.71 |
| 50 mg | 0.00036742 | 0.36742 | 367.42 |
| 100 mg | 0.00073484 | 0.73484 | 734.84 |
| 250 mg | 0.0018371 | 1.8371 | 1837.1 |
| 500 mg | 0.0036742 | 3.6742 | 3674.2 |
| 1 g | 0.0073484 | 7.3484 | 7348.4 |
| 2 g | 0.0146968 | 14.6968 | 14,696.8 |
| 5 g | 0.036742 | 36.742 | 36,742 |
| 10 g | 0.073484 | 73.484 | 73,484 |
| 25 g | 0.18371 | 183.71 | 183,710 |
| 50 g | 0.36742 | 367.42 | 367,420 |
| 100 g | 0.73484 | 734.84 | 734,840 |
| 250 g | 1.8371 | 1837.1 | 1837099.777369 |
| 500 g | 3.6742 | 3674.2 | 3674199.554738 |
Every row is the mass divided by 136.08406 g/mol. A balance reading to 1 mg resolves 7.348 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 |
|---|---|---|---|
| Calcium chloride | CaCl2 | 110.978 | 9.0108 |
| Magnesium sulfate | MgSO4 | 120.361 | 8.3083 |
| Ammonium sulfate | (NH4)2SO4 | 132.134 | 7.5681 |
| Monopotassium phosphate (this page) | KH2PO4 | 136.0841 | 7.3484 |
| Disodium phosphate | Na2HPO4 | 141.9573 | 7.0444 |
| Sodium sulfate | Na2SO4 | 142.0355 | 7.0405 |
| Potassium permanganate | KMnO4 | 158.0323 | 6.3278 |
Ordered by molar mass. The last column is 1000/M, which is the number a weighed gram actually gives you.