kJ/mol to kJ/g Converter
Energy per mole needs a weight before you can compare it with food or fuel. Divide the kJ/mol by the molar mass and you get kJ per gram.
Quick answer: divide the kJ/mol by the molar mass in g/mol. For example, 40.65 kJ/mol of water is 2.2565 kJ/g, 890 kJ/mol of methane is 55.49 kJ/g and 2,803 kJ/mol of glucose is 15.56 kJ/g.
The kJ/mol to kJ/g formula
One mole weighs the molar mass in grams (Wikipedia: Joule per mole).
So the energy for one gram is the kJ/mol divided by the molar mass.
Take 40.65 kJ/mol and a molar mass of 18.015. Divide to get 2.2565 kJ/g.
Take 890 kJ/mol and a molar mass of 16.04. Divide to get 55.49 kJ/g.
Energy per gram
The kJ/mol is energy for one mole (Wikipedia: Joule per mole). A mole of a compound weighs its molar mass in grams.
Food energy is quoted in kJ/g, such as 38 kJ/g for fats (Wikipedia: Specific energy).
One kJ/g equals one MJ/kg (Wikipedia: Specific energy).
How to calculate kJ/mol to kJ/g step by step
- Write down the energy in kJ/mol.
- Find the molar mass in g/mol.
- Divide the kJ/mol by the molar mass.
- The answer is in kJ/g.
Example 1: water vaporizing
- Water needs about 40.65 kJ/mol. Its molar mass is 18.015.
- 40.65 ÷ 18.015 = 2.2565.
- The energy is 2.2565 kJ/g.
40.65 kJ/mol = 2.2565 kJ/g
Example 2: methane burning
- Methane gives about 890 kJ/mol. Its molar mass is 16.04.
- 890 ÷ 16.04 = 55.49.
- The energy is 55.49 kJ/g.
890 kJ/mol = 55.49 kJ/g
Example 3: glucose burning
- Glucose gives about 2,803 kJ/mol. Its molar mass is 180.16.
- 2,803 ÷ 180.16 = 15.56.
- The energy is 15.56 kJ/g.
2,803 kJ/mol = 15.56 kJ/g
Moderate-density foods hold about 7 to 13 kJ/g, and high-density foods more than 13 kJ/g (Wikipedia: Specific energy).
kJ/mol to kJ/g chart
| Energy | Molar mass | Energy per gram |
|---|---|---|
| 40.65 kJ/mol | 18.015 g/mol | 2.2565 kJ/g |
| 890 kJ/mol | 16.04 g/mol | 55.4863 kJ/g |
| 2,803 kJ/mol | 180.16 g/mol | 15.5584 kJ/g |
| 393.5 kJ/mol | 44.01 g/mol | 8.9411 kJ/g |
| 100 kJ/mol | 18.015 g/mol | 5.5509 kJ/g |
| 6.01 kJ/mol | 18.015 g/mol | 0.3336 kJ/g |
| 285.8 kJ/mol | 18.015 g/mol | 15.8646 kJ/g |
| 436 kJ/mol | 2.016 g/mol | 216.2698 kJ/g |
The highlighted row, 40.65 kJ/mol for water, is the default of the calculator. Change the molar mass to try another substance.
A quick mental shortcut for kJ/mol to kJ/g
Round the molar mass and divide. So 100 kJ/mol at 20 g/mol is about 5 kJ/g.
Then fix the answer with the exact mass.
What kJ/g means
It is the energy in one gram of a material. It is easy to compare with food labels.
It equals MJ/kg (Wikipedia: Specific energy).
What a mole is
A mole is 6.022 × 10²³ particles. Its mass in grams is the molar mass.
Water is 18.015 g/mol.
Why mass matters
The same energy per mole is a different value per gram for each substance.
A light molecule gives more kJ/g than a heavy one.
Fuels and food
Fats are about 38 kJ/g. Proteins are about 16 kJ/g and carbohydrates about 17 kJ/g (Wikipedia: Specific energy).
Methane at 55.49 kJ/g is higher still.
Where to find molar mass
Add the atomic masses in the formula. For CO₂ that is 12.011 plus 2 × 15.999, which is 44.01.
Data tables list it too.
Phase changes
Water takes 6.01 kJ/mol to melt, which is 0.3336 kJ/g. It takes 40.65 kJ/mol to vaporize, which is 2.2565 kJ/g.
So boiling needs about 7 times more.
Bond and reaction values
Bond energies near 100 kJ/mol are about 5.55 kJ/g at the mass of water (Wikipedia: Joule per mole).
Per gram values change fast as the molar mass changes.
kJ/g back to kJ/mol
To go back, multiply the kJ/g by the molar mass.
So 2.2565 kJ/g at 18.015 g/mol is 40.65 kJ/mol.
Check your answer
Multiply your kJ/g answer by the molar mass. You should get your starting kJ/mol.
A bigger molar mass should give a smaller kJ/g.
Quick answers
| Input | Result |
|---|---|
| 6.01 kJ/mol at 18.015 g/mol | 0.3336 kJ/g |
| 40.65 kJ/mol at 18.015 g/mol | 2.2565 kJ/g |
| 100 kJ/mol at 18.015 g/mol | 5.5509 kJ/g |
| 393.5 kJ/mol at 44.01 g/mol | 8.9411 kJ/g |
| 890 kJ/mol at 16.04 g/mol | 55.4863 kJ/g |
| 2,803 kJ/mol at 180.16 g/mol | 15.5584 kJ/g |
Where this calculator is used
Chemistry students turn molar data into mass data.
Food scientists link reaction energy to labels.
Engineers compare fuels by weight.
Three fuels
Methane, at about 890 kJ/mol and 16.04 g/mol, gives 55.5 kJ/g. Ethanol, at about 1,367 kJ/mol and 46.07 g/mol, gives 29.7 kJ/g.
Glucose, at about 2,803 kJ/mol and 180.16 g/mol, gives 15.6 kJ/g.
Checking in reverse
Multiply the kJ/g by the molar mass. 55.5 kJ/g times 16.04 gives back about 890 kJ/mol.
Why it matters
Per-gram figures let you compare fuels by weight. Per-mole figures compare them by molecule count, so the two lists can rank fuels differently.
Reading the answer
A result of 55.5 kJ/g tells you that one gram of methane holds more energy than one gram of glucose, at 15.6 kJ/g. Gas fuels are light for each mole, so they score high per gram. Use the per-gram figure when weight matters, such as for a fuel tank.
Common mistakes to avoid
- Forgetting the molar mass. The answer depends on it.
- Multiplying instead of dividing. Per gram is smaller than per mole for most compounds.
- Using kg/mol. The mass must be in g/mol.
- Mixing up kJ/g and kcal/g. They differ by 4.184.
Related calculators
A melting example
Ice melts at 6.01 kJ/mol. That is 0.3336 kJ/g.
So a gram of ice takes about 333.6 J to melt.
A fuel example
Methane gives 55.49 kJ/g. A gram of glucose gives only 15.56 kJ.
Gas is the stronger fuel by weight.
A hydrogen example
Hydrogen burns to give 285.8 kJ per mole of water formed. That is 15.86 kJ for each gram of water.
The gram of hydrogen fuel gives about 141.8 kJ.
Frequently asked questions
How do I convert kJ/mol to kJ/g?
Divide the kJ/mol by the molar mass in g/mol.
Do I need the molar mass?
Yes. It links a mole to grams.
How many kJ/g is 40.65 kJ/mol of water?
About 2.2565 kJ/g.
What is the molar mass of water?
18.015 g/mol.
Is kJ/g the same as MJ/kg?
Yes.
How many kcal/g is 1 kJ/g?
About 0.239 kcal/g.
Sources
- Wikipedia: Joule per mole (en.wikipedia.org)
- Wikipedia: Specific energy (en.wikipedia.org)
