The Mole

• The?mole?is one of the seven SI base units
• It is used to measure the?amount of substance
• One mole is defined as follows:

The amount of substance that contains as many elementary entities as the number of atoms in 12 g of carbon-12

• This amount of substance is exactly 6.02214076 × 10²³?elementary entities (i.e. particles, atoms, molecules)
  • At IB level, this number can be rounded to 6.02 × 10²³
• One mole of gas contains a number of particles (atoms or molecules) equal to the?Avogadro constant
• Therefore, to calculate the number of particles?N?in a gas, knowing the number of moles?n, the following relationship must be used:

N = nN_A

• Where:
  • N?= number of gas particles
  • n?= number of moles of gas (mol)
  • NA?= 6.02 × 10²³?mol^–1?(Avogadro constant)

Molar Mass

• The?molar mass?M?of a substance is defined as the mass?m?of the substance divided by the amount (in moles) of that substance
• The molar mass is calculated as follows:

• Where:
  • M?= molar mass in g mol^–1
  • m?= mass in grams (g)
  • n?= number of moles (mol)

Step 1: Write down the relationship between the molar mass?M?and the mass?m

Step 2:?Rearrange the equation to find the mass?m?of?n?= 1 mol of nitrogen

m = Mn

m =?28.02 g mol^–1?× 1 mol

m?= 28.02 g

Step 3: Identify the number of nitrogen atoms in 1 mol of nitrogen

• • n?= 1 mol of nitrogen contains a number of?molecules?equal to the Avogadro constant
  • From the data booklet,?NA?=?6.02 × 10²³

Nmolecules?= nN_A

Nmolecules?=?6.02 × 10²³

• • Each nitrogen molecule contains 2 atoms of nitrogen
  • n?= 1 mol of nitrogen contains a number of?atoms?equal to twice the number of molecules

Natoms?= 2 ×?Nmolecules?= 2 × (6.02 × 10²³)

Natoms?= 1.2 ×?10²⁴

Step 4: Divide the mass of 1 mol of nitrogen?m?by the number of atoms in 1 mol of nitrogen?Natoms?in order to find the mass of a nitrogen atom

Mass of a nitrogen atom?= 2.3 × 10^–23?g