The Mole & The Avogadro Constant (Oxford AQA International A Level Chemistry)

Determine the number of atoms, molecules and the relative mass of 1 mole of:

1. Na

2. H₂ 

3. NaCl

Answer 1

• The relative atomic mass of Na is 23.0

• Therefore, 1 mol of Na has a mass of 23.0 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 1.0

• Since there are 2 H atoms in H₂, the mass of 1 mol of H₂ is (2 x 1.0) = 2.0 g mol^-1

• 1 mol of H₂ will contain 6.02 x 10²³ molecules of H₂

• Since there are 2 H atoms in H₂, 1 mol of H₂ will contain 2 x 6.02 x 10²³  = 1.204 x 10²⁴ H atoms

Answer 3

• The relative atomic mass of Na and Cl is 23.0 and 35.5 respectively

• Therefore, 1 mol of NaCl has a mass of (23.0 + 35.5) = 58.5 g mol^-1

• 1 mol of NaCl will contain 6.02 x 10²³ molecules of NaCl

• Since there is one Na and one Cl atom in NaCl, 1 mol of NaCl will contain 2 x 6.02 x 10²³  = 1.204 x 10²⁴ atoms in total

You only have to be able to work with Avogadro's constant, you will not be expected to recall the value.

1. What is the mass of 0.250 moles of zinc?

2. How many moles are in 2.64 g of sucrose, C₁₂H₁₁O₂₂  (M_r = 342.3)?

Answers:

1. 0.250 moles of zinc

• From the periodic table, the relative atomic / molar mass of Zn is 65.4 g mol^-1

• Mass = moles x molar mass

• Mass = 0.250 mol x 65.4 g mol^-1 = 16.4 g

2. 2.64 g of sucrose:

• The molar mass of sucrose is 342.3 g mol^-1

• Moles = mass ÷ molar mass

• Moles = 2.64 g ÷ 342.3 g mol^-1 = 7.71 x 10^-3 mol

Calculate the concentration, mol dm^-3, of the solution formed when 80.0 g of sodium hydroxide is dissolved in 500 cm³ of water.

Answer:

• The relative formula mass of NaOH is:

  • 23.0 + 16.0 + 1.0 = 40.0 g mol^-1

• So, 80 g of sodium hydroxide is:

  • Moles =

  • Moles = = 2 moles

• The volume in dm³ is:

  • = 0.5 dm³

• The concentration in mol dm^-3 is:

  • Concentration =

  • Concentration = = 4 mol dm^-3

Make sure that you use the correct units for moles, concentration, volume calculations.

These calculations have units of mol dm^-3 for concentration and dm³ for volume but questions will typically use cm³.