Linking Moles, Mass & Mr (CIE IGCSE Chemistry)

What is the mass of 0.250 moles of zinc?

Answer:

• From the Periodic Table, the relative atomic mass of Zn is 65
  • So, the molar mass is 65 g / mol
• The mass is calculated by moles x molar mass:
  • 0.250 mol x 65 g / mol = 16.25 g

How many moles are in 2.64 g of sucrose, C₁₂H₂₂O₁₁  (M_r = 342)?

Answer:

• The molar mass of sucrose is 342 g / mol
• The number of moles is found by mass ÷ molar mass:
  •  = 7.72 x 10^-3 mol

For magnesium chloride, MgCl₂, calculate the number of:

1. Molecules in 1 mole
2. Atoms in 1 mole
3. Chloride ions in 1 mole
4. Magnesium ions in 2 moles

Answers:

1. The formula is MgCl₂, so 1 mole of MgCl₂ is:
   • 1 x 6.02 x 10²³ = 6.02 x 10²³ molecules 
2. There are 3 atoms in MgCl₂, so 1 mole of MgCl₂ contains:
   • 3 x 6.02 x 10²³ = 18.06 x 10²³ atoms
3. There are 2 chloride ions in MgCl₂, so 1 mole of MgCl₂ contains:
   • 2 x 6.02 x 10²³ = 12.04 x 10²³ chloride ions
4. There is 1 magnesium ion in MgCl₂, so 2 mole of MgCl₂ contains:
   • 2 x (1 x 6.02 x 10²³) = 12.04 x 10²³ magnesium ions

In 15.7 g of water (M_r = 18):

1. How many molecules are there? 
2. How many atoms are there?

Answers:

1. The number of molecules:
   • The molar mass of water is 18 g / mol
   • The number of moles is found by mass ÷ molar mass
     • 15.7 g ÷ 18 g / mol = 0.872 mol
   • There are 6.02 x 10²³ molecules of water in 1 mole of water
   • So, in 0.872 moles of water, there are:
     • 6.02 x 10²³ x 0.872 = 5.25 x 10²³ molecules
2. The number of atoms:
   • In each molecule of water, there are 3 atoms (2 hydrogen atoms, one oxygen atom)
   • So, the number of atoms in 15.7 g =  3 x 5.25 x 10²³ = 1.58 x 10²⁴ atoms