• 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 10^23?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 10^23?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 10^23?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 10^24?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 10^23??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 10^24?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