Molar MassMolar mass of potassium dichromate (K2Cr2O7)
The molar mass of potassium dichromate, K2Cr2O7, is 294.18 g/mol. One mole of it, which is 6.022 × 1023 formula units, has a mass of 294.18 grams.
294.18 g/mol
2 × 39.098 + 2 × 51.996 + 7 × 15.999 = 294.181 · 11 atoms in the formula · also called potassium dichromate(VI)
About potassium dichromate
Potassium dichromate forms bright orange crystals. It is an oxidising agent and a primary standard in redox titrations, turning from orange to green as it reacts. Count carefully: two potassium, two chromium and seven oxygen atoms.
Other names: potassium dichromate(VI).
The working
The formula K2Cr2O7 has 2 potassium atoms, 2 chromium atoms and 7 oxygen atoms, 11 atoms in all. Multiply each count by the atomic weight of the element and add:
| Element | Atoms | Atomic weight | Total | Share by mass |
|---|---|---|---|---|
| Potassium (K) | 2 | 39.098 | 78.196 | 26.58% |
| Chromium (Cr) | 2 | 51.996 | 103.992 | 35.35% |
| Oxygen (O) | 7 | 15.999 | 111.993 | 38.07% |
2 × 39.098 + 2 × 51.996 + 7 × 15.999 = 294.181 g/mol
With the rounded atomic masses used in many school books (K = 39, Cr = 52, O = 16) the answer is 294 g/mol. The difference from 294.18 is too small to matter in most school sums.
Percentage composition
By mass, K2Cr2O7 is 26.58% potassium, 35.35% chromium and 38.07% oxygen. The largest share comes from oxygen. In a 100 g sample there are 26.6 g of potassium, 35.3 g of chromium and 38.1 g of oxygen.
Worked examples
Grams to moles. How many moles are in 100 g of K2Cr2O7? Divide the mass by the molar mass: 100 ÷ 294.18 = 0.3399 mol.
Moles to grams. What is the mass of 2 mol of K2Cr2O7? Multiply: 2 × 294.18 = 588.4 g.
Counting particles. 100 g contains 0.3399 mol × 6.022 × 1023 = 2.047 × 1023 formula units.
Making a solution. A 0.1 mol/L solution contains 0.1 × 294.18 = 29.42 g of K2Cr2O7 in each litre. For 250 mL, weigh 7.355 g and make it up to the mark with water.
Quick conversion tables
| Amount | Mass |
|---|---|
| 0.1 mol | 29.42 g |
| 0.5 mol | 147.1 g |
| 1 mol | 294.2 g |
| 2 mol | 588.4 g |
| 5 mol | 1471 g |
| Mass | Moles |
|---|---|
| 1 g | 0.003399 mol |
| 10 g | 0.03399 mol |
| 100 g | 0.3399 mol |
| 1 kg | 3.399 mol |
For other amounts use the grams and moles converter.
Questions people ask
What is the molar mass of K2Cr2O7?
The molar mass of potassium dichromate (K2Cr2O7) is 294.18 g/mol, or 294.181 g/mol before rounding, using IUPAC abridged atomic weights.
How many moles are in 100 g of K2Cr2O7?
100 ÷ 294.18 = 0.3399 mol.
What is the mass of one formula unit of K2Cr2O7?
Divide the molar mass by the Avogadro constant: 294.18 ÷ 6.022 × 1023 = 4.885 × 10-22 g.
What is the percentage of oxygen in K2Cr2O7?
Oxygen makes up 38.07% of the mass of K2Cr2O7: 7 × 15.999 ÷ 294.181 × 100.
Which values? This site uses the abridged standard atomic weights published by IUPAC (CIAAW), read on 12 November 2026. Your textbook or question paper may round them differently, for example H = 1 and O = 16, so its answer can differ slightly. In an exam, use the values the paper gives.