mole is the unit used to express the amount of matter. One mole is approximately 6.022 x 1023 particles. It is one of the fundamental quantities of the International System of Units (SI).
The term mol comes from the Latin word moles, which means “a mass”, and was introduced by the German chemist Wilhelm Ostwald.
Any substance can be measured in moles. We can use the mole to refer to something microscopic, like molecules, or something macroscopic, like seeds.
For example, 1 mole of water is equal to 6.022 x 1023 water molecules. Likewise, 1 mole of seeds is equal to 6.022 x 1023 seeds. Note that the number of elements in a mole is the same regardless of the entity being analyzed.
Using moles in chemical calculations is especially important for quantifying chemical species, such as atoms, ions, and molecules, involved in a chemical reaction. In addition, it is possible to make a proportion between an atomic scale and another measurable scale.
Mol and Avogadro's constant
The reference value used to standardize the amount of one mole is the mass of 12g of carbon-12.
Carbon-12 is made up of 6 protons, 6 neutrons and 6 electrons. It is the most abundant and stable isotope of the element carbon.
The Italian scientist Amedeo Avogadro (1776-1856) proposed that, under the same conditions of temperature and pressure, equal volumes of gases contain the same number of molecules.
For being a pioneer in the study of the relationship between mass in grams and atomic mass, when the number that makes the connection between these magnitudes was discovered in the 20th century, the magnitude of a mole was defined, in honor of the scientist, as the constant of Avogadro. Therefore:
1 mole = 6.02214179 × 1023 particles
Mol and mass calculations
THE atomic mass of the chemical elements is found in the Periodic Table. For example, the atomic mass of an atom of sodium (Na) is 23 g.
So 1 mol of sodium = 23 g = 6.022 x 1023 sodium atoms.
Note that mole, mass, and Avogadro's constant are related. If we know at least one of these values, we can determine the others using a simple rule of three, as in the following examples.
1. What is the mass contained in 2.5 mol of sodium (Na)?
1 mol 23 g of Na
2.5 mol x
x = 23. 2,5
x = 57.5 g of Na
2. How many moles are in 30 g of sodium (Na)?
1 mol 23 g of Na
x 30 g of Na
x = 30/23
x ≈ 1.3 mol of Na
3. How much matter is in 50 g of sodium (Na)?
23 g 6.022 x 1023
50 g x
x = 50. 6.022 x 1023/23
x = 13.09 x 1023 Na atoms
Check out the Periodic table complete and updated.
And the molar mass?
THE molar mass is the mass in grams contained in 1 mol of substance and its unit of measure is g/mol (grams per mol). Sodium, for example, has 23 g/mol.
The numerical value of the molar mass of a chemical substance is equivalent to the molecular mass (MM), that is, the sum of the atomic masses of the atoms that compose it.
We will use the water molecule (H2O) as an example and determine the mass of 1 mol of the substance.
1st step: count the number of atoms of chemical elements in the formula of the substance.
Water is made up of:
Oxygen (O): 1 atom
Hydrogen (H): 2 atoms
2nd step: Consult the Periodic Table for the atomic mass of the elements.
Note: to facilitate understanding, we will use approximate values here.
Oxygen (O): 16 u
Hydrogen (H): 1 u
3rd step: multiply the masses of the elements by the number of atoms in the substance.
Oxygen (O): 1 x 16 u = 1 x 16 u
Hydrogen (H): 2 x 1 u = 2 u
4th step: Add the atomic masses and determine the molecular mass.
MMWater: 16 u + 2 u = 18 u
Therefore, the molecular mass of water is 18 u and the molar mass is 18 g/mol. This means that in one mole there are 6.022 x 1023 water molecules, which corresponds to 18 grams.
Therefore, to determine the number of moles we need to know the mass and chemical composition of the substance.
Now, let's solve some more examples relating the quantities mol, mass and quantity of particles.
1. What is the mass contained in 3 moles of water (H2O)?
1 mol 18 g of H2O
3 moles x
x = 18. 3
x = 54 g of H2O
2. How many moles are there in 80 g of water (H2O)?
1 mol 18 g of H2O
x 80 g of H2O
x = 80/18
x ≈ 4.44 mol of H2O
3. What is the amount of matter in 20 g of water (H2O)?
18 g 6.022 x 1023
20 g x
x = 20. 6,022 x 1023/18
x = 6.69 x 1023 H molecules2O
Learn more about molecular mass.
Relationship between mole and molar volume
At STP, under normal conditions of temperature (273 K) and pressure (1 am), a gas occupies a volume of 22.4 L. This value is the molar volume of gases.
As Avogadro proposed, the volume occupied by gases, regardless of their composition, is related to the number of molecules. So, even if we have two different gases trapped in containers, if the volume is the same, the two flasks have the same amount of molecules.
For example, for the gases oxygen and hydrogen we have the following relationship:
1 mol of hydrogen (H2) = 22.4 L = 2 g = 6.022 x 1023 H molecules2
1 mole of oxygen (O2) = 22.4 L = 32 g = 6.022 x 1023 O molecules2
Note that 1 mol of any substance in the gaseous state occupies a volume of 22.4 L, but the mass will be different because the compositions of the gases are different.
Learn more about Avogadro's Law.
