Moles to grams for monopotassium phosphate is a multiplication by 136.08406 g/mol. A mole weighs 136.0841 g, 100 mmol weighs 13.6084 g, 10 mmol weighs 1.36084 g and a millimole is 136.084 mg. Those four figures cover most of what gets weighed out in practice, and the table below extends them from a micromole up to ten moles.
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.
Two habits make the weighed mass match the calculation. Weigh into a container you have tared rather than onto paper, because what stays behind on paper is a real loss — on a 100 mg weighing it is easily a percent. And when the target is under about 20 mg, weigh ten times that and dilute, since the absolute error of the balance does not shrink with the sample.
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.
Purity is the usual gap between the calculation and the balance. A reagent sold at 98% means 2.7217 g of a nominal 136.0841 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
n- The amount of monopotassium phosphate you need, in moles
136.08406- The molar mass of monopotassium phosphate in g/mol, derived from its formula
m- The mass to weigh out, in grams
How it works, step by step
- Enter the amount you need, in moles.
- It is multiplied by 136.08406 g/mol.
- The result is the mass to weigh out, also given in milligrams.
- Weigh into a tared container; anything left in the boat is a real loss from the amount you calculated.
Worked examples
5 mmol of monopotassium phosphate in grams
5 mmol × 136.0841 g/mol = 0.68042 g, which is 680.42 mg. Doubling the amount doubles the mass to 1.36084 g, because the relationship is a plain proportion once the molar mass is fixed.
250 mmol of monopotassium phosphate in grams
250 mmol × 136.0841 g/mol = 34.021 g, which is 34,021 mg. Doubling the amount doubles the mass to 68.042 g, because the relationship is a plain proportion once the molar mass is fixed.
2 mol of monopotassium phosphate in grams
2 mol × 136.0841 g/mol = 272.168 g, which is 272,168 mg. Doubling the amount doubles the mass to 544.336 g, because the relationship is a plain proportion once the molar mass is fixed.
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 many grams is one mole of monopotassium phosphate?
136.08406 g, by definition of the molar mass. Two moles is 272.1681 g and half a mole is 68.04203 g.
How many grams is 0.1 mol, and how many milligrams is 1 mmol?
0.1 mol of monopotassium phosphate is 13.6084 g. One millimole is 136.084 mg and one micromole is 136.084 µg, which is the range most bench work sits in.
How do I weigh out an amount too small for my balance?
Weigh a larger mass and dilute it. To get 1.361 mg of monopotassium phosphate, weigh 13.61 mg into 100 mL, then take 10 mL of that: the absolute error of the balance stays the same while the amount delivered falls by a factor of ten.
Should I use the hydrate or the anhydrous molar mass?
This page is the anhydrous form, KH2PO4, at 136.08406 g/mol. If your jar is the other form the molar mass differs and so will every mass you weigh, so check the label before using the figure. Salts that are sold as hydrates weigh more per mole because the water is part of the crystal.
Moles to grams 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.
| Wanted | Weigh (g) | Weigh (mg) | With 2% over (g) |
|---|---|---|---|
| 1 µmol | 0.000136084 | 0.136084 | 0.000138806 |
| 10 µmol | 0.00136084 | 1.36084 | 0.00138806 |
| 100 µmol | 0.0136084 | 13.6084 | 0.0138806 |
| 1 mmol | 0.136084 | 136.084 | 0.138806 |
| 10 mmol | 1.36084 | 1360.84 | 1.38806 |
| 50 mmol | 6.8042 | 6804.2 | 6.94029 |
| 100 mmol | 13.6084 | 13,608.4 | 13.8806 |
| 250 mmol | 34.021 | 34,021 | 34.7014 |
| 500 mmol | 68.042 | 68,042 | 69.4029 |
| 750 mmol | 102.063 | 102,063 | 104.104 |
| 1 mol | 136.084 | 136,084 | 138.806 |
| 2 mol | 272.168 | 272,168 | 277.611 |
| 5 mol | 680.42 | 680,420 | 694.029 |
| 10 mol | 1360.84 | 1360840.619980 | 1388.06 |
Weigh is the amount multiplied by 136.08406 g/mol. The last column carries a 2% allowance, which is what you take when some of the solid will be left in the weighing boat.
| 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.