The molar mass of potassium dichromate, K2Cr2O7, is 294.1818 g/mol. That single number is what connects a mass you can weigh to an amount of substance you can react: one mole is 294.1818 g, and a gram of it is 2.047e+21 formula units. The calculator turns any weighed mass into that count, and reports the amount in moles, millimoles and the mass of a single formula unit alongside it.
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.
The numbers a bench actually uses sit at the millimole scale. One millimole of potassium dichromate is 294.182 mg and one micromole is 294.182 µg, so a balance reading to a milligram places you within 3.399 µmol. A single formula unit weighs 4.8850e-22 g, which is why the count in the gauge runs into the sextillions for any mass you can see.
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
m- The mass of potassium dichromate in grams
294.1818- The molar mass of potassium dichromate in g/mol, derived from its formula
N- The number of formula units that mass contains
How it works, step by step
- Enter a mass of potassium dichromate in grams.
- It is divided by the molar mass, 294.1818 g/mol, giving the amount in moles.
- That amount is multiplied by the Avogadro constant, 6.02214076 × 10²³ per mole, to give the number of formula units.
- The panel also reports the amount in moles and millimoles and the mass of one formula unit.
Worked examples
0.5 g of potassium dichromate as a molecule count
0.5 g divided by 294.1818 g/mol is 0.00169963 mol, and multiplying by the Avogadro constant gives 1.024e+21 formula units of potassium dichromate. The same mass is 1.6996 mmol, which is the figure a reaction is actually planned in.
5 g of potassium dichromate as a molecule count
5 g divided by 294.1818 g/mol is 0.0169963 mol, and multiplying by the Avogadro constant gives 1.024e+22 formula units of potassium dichromate. The same mass is 16.996 mmol, which is the figure a reaction is actually planned in.
50 g of potassium dichromate as a molecule count
50 g divided by 294.1818 g/mol is 0.169963 mol, and multiplying by the Avogadro constant gives 1.024e+23 formula units of potassium dichromate. The same mass is 169.96 mmol, which is the figure a reaction is actually planned in.
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 molecules are in one gram of potassium dichromate?
About 2.047e+21. One gram is 0.00339926 mol, and each mole contains 6.02214076 × 10²³ formula units by definition of the mole, so the count is that constant divided by 294.1818.
How much does a single molecule of potassium dichromate weigh?
4.8850e-22 g, which is 294.1818 daltons. It is the molar mass divided by the Avogadro constant, and the number in daltons is numerically the same as the molar mass in g/mol — that equivalence is what makes the mole convenient.
Is molar mass the same as molecular weight?
They are the same number in ordinary use but not the same quantity. Molecular weight, properly relative molecular mass, is a ratio and has no unit: for potassium dichromate it is 294.1818. Molar mass carries grams per mole: 294.1818 g/mol. Because the mole is defined so those agree, you can read one off the other.
Why is the molar mass not a whole number?
Because standard atomic weights are averages over the isotopes found in nature. Oxygen is quoted as 15.999 rather than a whole number for that reason, and the same applies to the other elements here, which is why potassium dichromate comes out at 294.1818 g/mol rather than 294.
Molar mass, composition and mole equivalents of 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.
| Amount | Mass (g) | Mass (mg) | Formula units |
|---|---|---|---|
| 1 µmol | 0.000294182 | 0.294182 | 6.022e+17 |
| 10 µmol | 0.00294182 | 2.94182 | 6.022e+18 |
| 100 µmol | 0.0294182 | 29.4182 | 6.022e+19 |
| 1 mmol | 0.294182 | 294.182 | 6.022e+20 |
| 10 mmol | 2.94182 | 2941.82 | 6.022e+21 |
| 50 mmol | 14.7091 | 14,709.1 | 3.011e+22 |
| 100 mmol | 29.4182 | 29,418.2 | 6.022e+22 |
| 250 mmol | 73.5455 | 73,545.4 | 1.506e+23 |
| 500 mmol | 147.091 | 147,091 | 3.011e+23 |
| 750 mmol | 220.636 | 220,636 | 4.517e+23 |
| 1 mol | 294.182 | 294,182 | 6.022e+23 |
| 2 mol | 588.364 | 588,364 | 1.204e+24 |
| 5 mol | 1470.91 | 1470909.000000 | 3.011e+24 |
| 10 mol | 2941.82 | 2941818.000000 | 6.022e+24 |
Mass is the amount multiplied by 294.1818 g/mol. The last column is that amount multiplied by the Avogadro constant, 6.02214076 × 10²³ per mole.
| 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.