Moles to grams for potassium dichromate is a multiplication by 294.1818 g/mol. A mole weighs 294.1818 g, 100 mmol weighs 29.4182 g, 10 mmol weighs 2.94182 g and a millimole is 294.182 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. Potassium dichromate is 2 × 39.0983 (K) + 2 × 51.9961 (Cr) + 7 × 15.999 (O), which is 294.1818 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: 7 of the 11 atoms in a formula unit are oxygen, and they account for 111.993 g of the 294.1818 g, or 38.07% 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.
A primary standard for oxidation titrations because it can be dried and weighed exactly. It is a confirmed carcinogen and is handled accordingly. At 294.1818 g/mol it is heavier than 32 of the 34 salts covered here, and that ranking matters more than it looks: zinc sulfate heptahydrate has a molar mass of 287.541 g/mol, so a gram of it contains 2.31% more formula units than a gram of potassium dichromate. 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 5.8836 g of a nominal 294.1818 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 potassium dichromate you need, in moles
294.1818- The molar mass of potassium dichromate 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 294.1818 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 potassium dichromate in grams
5 mmol × 294.1818 g/mol = 1.47091 g, which is 1470.91 mg. Doubling the amount doubles the mass to 2.94182 g, because the relationship is a plain proportion once the molar mass is fixed.
250 mmol of potassium dichromate in grams
250 mmol × 294.1818 g/mol = 73.5455 g, which is 73,545.4 mg. Doubling the amount doubles the mass to 147.091 g, because the relationship is a plain proportion once the molar mass is fixed.
2 mol of potassium dichromate in grams
2 mol × 294.1818 g/mol = 588.364 g, which is 588,364 mg. Doubling the amount doubles the mass to 1176.73 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 potassium dichromate (K2Cr2O7)?
It is 294.1818 g/mol. That is the sum of the standard atomic weights of every atom in the formula: 2 × 39.0983 (K) + 2 × 51.9961 (Cr) + 7 × 15.999 (O). One mole of potassium dichromate therefore weighs 294.1818 g, and a gram of it is 3.3993 mmol.
What percentage of potassium dichromate is oxygen?
38.07% by mass. Each formula unit contains 7 oxygen atoms contributing 111.993 g of the 294.1818 g total, so 100 g of potassium dichromate contains 38.07 g of oxygen and a kilogram contains 380.69 g of it.
How many grams is one mole of potassium dichromate?
294.1818 g, by definition of the molar mass. Two moles is 588.3636 g and half a mole is 147.0909 g.
How many grams is 0.1 mol, and how many milligrams is 1 mmol?
0.1 mol of potassium dichromate is 29.4182 g. One millimole is 294.182 mg and one micromole is 294.182 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 2.942 mg of potassium dichromate, weigh 29.42 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, K2Cr2O7, at 294.1818 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 potassium dichromate
| Element | Atoms | Atomic weight | Contribution (g/mol) | By mass |
|---|---|---|---|---|
| Oxygen (O) | 7 | 15.999 | 111.993 | 38.07% |
| Chromium (Cr) | 2 | 51.9961 | 103.9922 | 35.35% |
| Potassium (K) | 2 | 39.0983 | 78.1966 | 26.58% |
| Total — one mole of potassium dichromate | 294.1818 | 100% |
Standard atomic weights, IUPAC 2021. The contribution column is atoms × atomic weight, and the total is the molar mass this page uses: 294.1818 g/mol.
| Wanted | Weigh (g) | Weigh (mg) | With 2% over (g) |
|---|---|---|---|
| 1 µmol | 0.000294182 | 0.294182 | 0.000300065 |
| 10 µmol | 0.00294182 | 2.94182 | 0.00300065 |
| 100 µmol | 0.0294182 | 29.4182 | 0.0300065 |
| 1 mmol | 0.294182 | 294.182 | 0.300065 |
| 10 mmol | 2.94182 | 2941.82 | 3.00065 |
| 50 mmol | 14.7091 | 14,709.1 | 15.0033 |
| 100 mmol | 29.4182 | 29,418.2 | 30.0065 |
| 250 mmol | 73.5455 | 73,545.4 | 75.0164 |
| 500 mmol | 147.091 | 147,091 | 150.033 |
| 750 mmol | 220.636 | 220,636 | 225.049 |
| 1 mol | 294.182 | 294,182 | 300.065 |
| 2 mol | 588.364 | 588,364 | 600.131 |
| 5 mol | 1470.91 | 1470909.000000 | 1500.33 |
| 10 mol | 2941.82 | 2941818.000000 | 3000.65 |
Weigh is the amount multiplied by 294.1818 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 |
|---|---|---|---|
| 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 (this page) | K2Cr2O7 | 294.1818 | 3.3993 |
| Aluminium sulfate | 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.