Understanding Chemical Reactions

1
Understanding Chemical Reactions

• Lesson Calculations in Chemistry 2

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Page 175 Q 4

• One mole of ZnO has a RAM of
• 65 16 81g
• 0.2 moles will have a mass of
• 0.2 X 81 16.2 g

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Page 175 Q 4

• b) 1 mole of H2S has a mass of
• (2x1) 32 34g
• 2.5 moles will have a mass of
• 2.5 X 34 85 g

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Page 175 Q 4

• c) 1 mole of CuSO4 has a mass of
• 63.5 32 (16 x 4) 259.5g
• 0.45 moles will have a mass of
• 0.45 X 259.5 116.8 g

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Calculating formulae

• Page 178 of Chemistry text

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Working out the formula of magnesium oxide

• A student heated some Mg as shown.
• When Mg burns in air it combines with oxygen to
  make magnesium oxide.

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Working out the formula of magnesium oxide

• Step 1 - Results
• Mass of crucible lid Mg before heating
  25.24g
• Mass of crucible and lid 25.00g
• Therefore, mass of Mg 0.24g

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Working out the formula of magnesium oxide

• Step 2 - Results
• Mass of crucible lid magnesium oxide after
  heating 25.40g
• Mass of crucible lid Mg before heating
  25.24g
• Therefore, mass of oxygen in magnesium oxide
  0.16g

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Working out the formula of magnesium oxide

• Step 3 Change the masses into moles
• Magnesium 0.24 / 24 0.01 mole
• Oxygen 0.16 / 16 0.01 mole

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Working out the formula of magnesium oxide

• Step 4 Work out the ratio of moles
• Mg O
• 0.01 0.01
• 1 1
• Therefore the formula of magnesium oxide is MgO

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One for you!

• A compound of nitrogen and hydrogen was broken
  down into its elements.
• It was found that 1.4g of nitrogen had combined
  with 0.3g of hydrogen in the compound.
• What was the formula of the compound?
• (R.A.M.s N 14, H 1)

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One for you!

• Step 1 work out the number of moles
• Moles of N 1.4 / 14 0.1
• Moles of H 0.3 / 1 0.3

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One for you!

• Step 2 work out the ratio of the number of
  moles to the lowest whole numbers
• N H
• 0.1 0.3
• 1 3
• Therefore there is 3 times as many H atoms as N
  atoms.
• Its formula must be NH3

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Reacting iron with copper sulphate

• A more reactive metal will displace a less
  reactive metal from its solution
• Iron copper sulphate iron sulphate
  copper
• Fe(s) CuSO4(aq)
  FeSO4(aq) Cu(s)
• How much copper will 0.5g of iron produce?

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Reacting iron with copper sulphate

• Fill your beaker with copper sulphate solution.
• Add precisely 5g of iron to the beaker and stir
  gently for 4 minutes.
• Filter the mixture making sure all the solid is
  removed from the beaker rinse beaker with water
  if needed.

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Reacting iron with copper sulphate

• Rinse the filter paper with propanone to dry the
  copper.
• When dry re-weigh the solid.

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• 1 mole of Fe should produce 1 mole of Cu
• 5 g of Fe is 5 / 56 0.09 moles
• 0.09 moles of Fe should produce 0.09 moles of Cu
• 0.09 moles of Cu will have a mass of
• 0.09 x 64 5.7 g