To handle chemical reactions quantitatively, we need a unit that efficiently counts the numbers of atoms and molecules. That unit is the amount of substance (mol).
Basic Knowledge
Atoms and molecules are extremely small, so it is impossible to count them one by one. Therefore, in chemistry, 6.02×1023 particles are counted as one group, defined as 1 mol. This number is called the Avogadro constant NA.
For example, if 6.02×1023 H2O molecules are gathered together, they constitute 1 mol. The unit of the Avogadro constant, which represents the number of particles per mol, is /mol.
The relationship “number of particles = amount of substance × Avogadro constant” holds, allowing us to calculate the number of particles using the amount of substance.
Amount of substance (A quantity expressing the number of particles in mol units. Symbol n)
Avogadro constant (Number of particles per 1 mol NA = 6.02×1023 /mol)
1 mol (An amount containing 6.02×1023 particles)
Type of particle (Atoms, molecules, ions, electrons, etc. Always state which particles are involved)
Avogadro (An Italian scientist. Proposed the molecular theory in 1811)
Further Exploration (Background and Significance)
Why do we use such a large number as 6.02×1023? The mass of one atom is extremely small, so a suitable number that can be handled in grams is on the order of 1023. Specifically, it was defined so that the number of carbon atoms contained in 12 g of carbon 12C would be approximately 6.02×1023 (it is now a fixed value in the SI system).
Using mol, the coefficients in a chemical equation can be treated directly as ratios of amounts of substance, as in “2 mol of H2 reacts with 1 mol of O2 to produce 2 mol of H2O.” This is why mol is central to chemical calculations.
- 1 mol = 6.02×1023 particles
- Avogadro constant NA ≒ 6.02×1023 /mol
- Number of particles = amount of substance (mol) × NA
- Amount of substance = number of particles ÷ NA
- Always check “which particles” are involved (atoms or molecules)
- H2O 1 mol = 6.02×1023 water molecules = 1.204×1024 hydrogen atoms
- Coefficients in a chemical equation ≒ ratio of amounts of substance
Points of Caution (Common Confusions)
① The amount of substance of atoms and the amount of substance of molecules are different. H2O 1 mol contains 2 mol of H atoms. ② When finding the amount of substance of ions, count the number of formula units. ③ The unit of the Avogadro constant is /mol, so it is not dimensionless. ④ In recent usage, the Avogadro “constant” (a physical quantity) and Avogadro “number” (a numerical value) are distinguished.
Practice
- There are 3.01×1023 H2O molecules. How many mol is this?
- Find the number of oxygen molecules contained in 2.0 mol of O2.
- State the amount of substance of hydrogen atoms contained in 1.0 mol of H2O.