The Mole (Leaving Cert Chemistry): Revision Notes

The Mole

The Mole and Avogadro's Constant

• Avogadro's constant: $6.02 \times 10^{23}$, representing the number of particles (atoms, molecules, or ions) in one mole of any substance.
• Particles may be atoms (for elements) or molecules (for compounds).
• The mole (mol) is the SI unit for the amount of substance.
• One mole contains exactly Avogadro's number of particles.

Standard Temperature and Pressure (STP)

• Standard temperature and pressure (STP) are conditions used as a reference for gas calculations.
  • Temperature: 273 K (0°C)
  • Pressure: 1 atmosphere (101.3 kPa)
• Under STP, 1 mole of any gas occupies a volume of 22.4 litres.
  • This is known as the molar volume at STP.

Molar Mass and Relative Molecular Mass (Mr)

• Molar mass (g/mol): The mass of one mole of a substance in grammes. It is numerically equal to the substance's relative molecular mass (Mr).
• Relative molecular mass (Mr): The sum of the relative atomic masses of all atoms in a molecule.

Converting Between Units

Moles to Mass (Grammes):

To calculate mass from moles:

$$\text{Mass (g)} = \text{Moles} \times \text{Molar Mass (g/mol)}$$

Mass to Moles:

To calculate moles from mass:

$$\text{Moles} = \frac{\text{Mass (g)}}{\text{Molar Mass (g/mol)}}$$

Moles to Number of Particles:

To calculate the number of particles from moles:

$$\text{Number of Particles} = \text{Moles} \times \text{Avogadro's Constant}$$

Volume of Gas to Moles (at STP):

To calculate moles from the volume of gas:

$$\text{Moles} = \frac{\text{Volume (litres)}}{22.4 \, \text{L/mol}}$$

Moles to Number of Atoms (Molecular Species):

For molecules, multiply by the number of atoms in each molecule.

Determination of Mr Using a Mass Spectrometer

In a mass spectrometer, the relative molecular mass (Mr) of a substance can be determined through the following steps:

1. Vaporisation: The sample is vaporised.
2. Ionisation: The vaporised atoms are ionised, usually by losing an electron.
3. Acceleration: The ions are accelerated through an electric field.
4. Separation: The ions are separated based on their mass-to-charge ratio.
5. Detection: The ions are detected, and their abundance is measured to determine the relative molecular mass.