1.1.4 The Mole Concept

• The Avogadro constant (N_A or L) is the number of particles equivalent to the relative atomic mass or molecular mass of a substance in grams
  • The Avogadro constant applies to atoms, molecules and ions
  • The value of the Avogadro constant is 6.02 x 10²³ g mol^-1
• The mass of a substance with this number of particles is called the molar mass
  • One mole of a substance contains the same number of fundamental units as there are atoms in exactly 12.00 g of ¹²C
  • If you had 6.02 x 10²³ atoms of carbon-12 in your hand, you would have a mass of exactly 12.00 g
  • One mole of water would have a mass of (2 x 1.01 + 16.00) = 18.02 g

Answer 1:

• • The relative atomic mass of Na is 22.99
  • Therefore, 1 mol of Na has a mass of 22.99 g mol^-1
  • 1 mol of Na will contain 6.02 x 10²³ atoms of Na (Avogadro’s constant)

Answer 2:

• • The relative atomic mass of H is 1.01
  • Since there are 2 H atoms in H₂, the mass of 1 mol of H₂ is (2 x 1.01) 2.02 g mol^-1
  • 1 mol of H₂ will contain 6.02 x 10²³ molecules of H₂
  • However, since there are 2 H atoms in each molecule of H₂, 1 mol of H₂ molecules will contain 1.204 x 10²⁴ H atoms

Answer 3:

• • The relative atomic masses of Na and Cl are 22.99 and 35.45 respectively
  • Therefore, 1 mol of NaCl has a mass of (22.99 + 35.45) 58.44 g mol^-1
  • 1 mol of NaCl will contain 6.02 x 10²³  formula units of NaCl
  • Since there is both an Na and a Cl atom in NaCl, 1 mol of NaCl will contain 1.204 x 10²⁴ atoms in total

Relative atomic mass, A_r

• The relative atomic mass (A_r) of an element is the weighted average mass of one atom compared to one twelfth the mass of a carbon-12 atom
• The relative atomic mass is determined by using the weighted average mass of the isotopes of a particular element
• The A_r has no units as it is a ratio and the units cancel each other out

Relative isotopic mass

• The relative isotopic mass is the mass of a particular atom of an isotope compared to one twelfth the mass of a carbon-12 atom
• Atoms of the same element with a different number of neutrons are called isotopes
• Isotopes are represented by writing the mass number as ²⁰Ne, or neon-20 or Ne-20
  • To calculate the average atomic mass of an element the percentage abundance is taken into account
  • Multiply the atomic mass by the percentage abundance for each isotope and add them all together
  • Divide by 100 to get average relative atomic mass
  • This is known as the weighted average of the masses of the isotopes

Relative molecular mass, M_r

• The relative molecular mass (M_r) is the weighted average mass of a molecule compared to one twelfth the mass of a carbon-12 atom
• The M_r has no units

• The M_r can be found by adding up the relative atomic masses of all atoms present in one molecule
• When calculating the M_r the simplest formula for the compound is used, also known as the formula unit
  • E.g. Silicon dioxide has a giant covalent structure, but the simplest formula (the formula unit) is SiO₂

Relative formula mass, M_r 

• The relative formula mass (M_r) is used for compounds containing ions
• It has the same units and is calculated in the same way as the relative molecular mass
• In the table above, the M_r for potassium carbonate, calcium hydroxide and ammonium sulfate are relative formula masses