Linking Moles, Mass & Mr (Cambridge (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

Always show your workings in calculations as its easier to check for errors and you may pick up credit if you get the final answer wrong.

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