Linking Moles, Mass & Mr (CIE IGCSE Chemistry)

Calculating Moles

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• Although elements and chemicals react with each other in molar ratios, in the laboratory we use digital balances and grams to measure quantities of chemicals as it is impractical to try and measure out moles
• Therefore we have to be able to convert between moles and grams
• We can use the following formula to convert between moles, mass in grams and the molar mass
• The mass of 1 mole of a substance is known as the molar mass
• For an element, it is the same as the relative atomic mass written in grams
• For a compound it is the same as the relative formula mass or relative molecular mass in grams

Formula triangle for moles, mass and molar mass

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 = 16.3 g

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

   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