Linking Moles, Mass & Mr (Cambridge (CIE) O Level Chemistry): Revision Note

What is the mass of 0.250 moles of zinc?

Answer

• From the Periodic Table, the relative atomic mass of Zn is 65.4

• So, the molar mass is 65.4 g/mol

• The mass is calculated by moles x molar mass

• This comes to 0.250 mol x 65.4 g/mol = 16.35 g

How many moles are in 2.64 g of sucrose, C₁₂H₂₂O₁₁  (M_r = 342.3)?

Answer

• The molar mass of sucrose is 342.3 g/mol

• The number of moles is found by mass ÷ molar mass

• This comes to  2.64 g ÷ 342.3 g/mol = 7.71 x 10^-3 mol

In 15.7 g of water (M_r = 18):

1. How many molecules are there? 

2. How many atoms are there?

Answer - part a

• The molar mass of water is 18 g/mol

• The number of moles is found by mass ÷ molar mass

• This comes to 15.7 g ÷ 18 g/mol = 0.872 mol

• There are 6.02 x 10²³ molecules of water in 1 mole of water

• In 0.872 moles of water, there are 6.02 x 10²³ x 0.872 = 5.25 x 10²³ molecules

Answer - part b

• In each molecule of water there are 3 atoms (2 hydrogen atoms, one oxygen atom)

• The number of atoms in 15.7 g =  3 x 5.25 x 10²³ = 1.58 x 10²⁴ atoms

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.