Chapter 6 · Chemical Composition
6.3Converting between Moles and Number of Atoms
10 min · two checks
Predict
How many atoms are in 0.250 mol of copper?
The idea
- Convert moles to particles and particles to moles.
- Keep the identity of the particle explicit.
The equality is 1 mol = 6.022 × 10²³ particles. As a factor it points either way. Moles to atoms: multiply. Atoms to moles: divide. The arithmetic is ordinary scientific notation. What must not get casual is the word after the number. 1 mol of O₂ contains 6.022 × 10²³ molecules, and twice that many oxygen atoms, because each molecule has two atoms. The conversion counts whatever you said “mole of.”
Significant figures follow the given measurement. Avogadro’s number is known more precisely than most class data, so it rarely limits a three-figure problem. Write the unit mol or atoms in the factor so a later step can cancel it.
Keep these
- particles = moles × 6.022 × 10²³.
- moles = particles / 6.022 × 10²³.
- Specify atoms, molecules, or formula units. They are not interchangeable inside a formula.
Worked path
A sample contains 2.50 mol of N₂. How many nitrogen molecules is that, and how many nitrogen atoms?
Observe
2.50 mol of molecules. Each molecule has 2 atoms.