Moles Determination (OCR AS Chemistry)

• There are a series of suggested practicals for PAG 1 - Moles Determination
  • PAG 1.1 - Determination of the composition of copper(II) carbonate basic
  • PAG 1.2 - Determination of the relative atomic mass of magnesium
  • PAG 1.3 - Determination of the formula of magnesium oxide
• All three suggested practicals require the number of moles to be calculated from experimental data to determine the final answer
• This can be done by:
  • Measuring gas volumes
    • In these experiments, a reaction occurs which produces a gas
    • The number of moles of gas produced is calculated
    • The stoichiometric ratio of the chemical reaction is then used to determine the final answer
    • For this style of experiment, the final answer could be the composition of copper(II) carbonate or the relative atomic mass of magnesium
    • Other practicals can be used in place of the suggested practicals so you wont be required to apply your knowledge of these specific reactions to other examples in questions 
  • Measuring changes in mass
    • In these experiments, a reaction occurs which produces an increase or decrease in mass
    • The suggested practical that goes with this is determining the formula of magnesium oxide
    • A known mass of magnesium is combusted and the mass of the magnesium oxide product is measured
    • The number of moles of magnesium and oxygen are calculated to determine the ratio of each element in the formula of the magnesium oxide
    • A similar practical can be performed on hydrated salts, e.g. CuSO₄•xH₂O, by heating them to a constant mass and using the decreases in mass to calculate the number of water of crystallisations - but this is not a suggested practical

PAG 1.1 - Determination of the composition of copper(II) carbonate basic

1. Set up the apparatus using a measuring cylinder sitting in water bath connected to a conical flask
2. Measure a known mass of CuCO₃.Cu(OH)₂ and add it to the conical flask
3. Add an excess of sulfuric acid into the conical flask and immediately insert the stopper. The gas produced will collect in the measuring cylinder
4. Record the final volume of carbon dioxide in the measuring cylinder

Equipment set up for gas collection by displacement

Calculations

• We can find the number of moles, n, of CO₂ produced using the measured volume, v, with the following equation:
  •  or 
  • Remember: 24.0 dm³ is the volume of 1 mole of gas
  • This calculation is only correct if the reaction is completed at room temperature and pressure (101 kPa or 1 atmosphere)
  • If the calculation is completed under conditions that are not RTP, then a rearranged form of the ideal gas equation should be used:
  • 

• We can use the overall equation for the reaction to figure out the number of moles of copper carbonate that reacted

   CuCO₃ + H₂SO₄ → CuSO₄ + H₂O + CO₂

• Molar ratio for CuCO₃ : CO₂ is 1:1
• The mass, m, of CuCO₃ can then be determined using:
  • m = n x M_r  

• As our original sample contained CuCO₃.Cu(OH)₂ we need to determine the percentage by mass of CuCO₃ present in the original sample

Source of errors

• Some carbon dioxide may escape from the conical flask before inserting the bung
  • This can be avoided by inserting the bung as soon as the acid is poured into the conical flask
• Some of the copper carbonate may not react
  • Swirl the conical flask to ensure the contents are sufficiently mixed and the reaction goes to completion
• Carbon dioxide may dissolve in the water
  • A gas syringe can be used to measure the volume of CO₂ instead
• Some copper carbonate may remain on the weighing apparatus and not be transferred to the conical flask
  • Weigh the weighing apparatus with and without and copper carbonate on it 
  • The difference between these gives the exact amount added to the conical flask

PAG 1.2 - Determination of the relative atomic mass of magnesium

• This practical is completed in a similar way to PAG 1.1 - Determination of the composition of copper(II) carbonate basic in terms of equipment
• Set up the apparatus using a measuring cylinder sitting in water bath connected to a conical flask
• Measure a known mass of magnesium and add it to the conical flask
• Add an excess of sulfuric acid into the conical flask and immediately insert the stopper. The gas produced will collect in the measuring cylinder
• Record the final volume of hydrogen gas in the measuring cylinder

Calculations

• We can find the number of moles, n, of H₂ produced using the measured volume, v, with the following equation:
  • 
  • Remember: 24 dm³ is the volume of 1 mole of gas

• We can use the overall equation for the reaction to figure out the number of moles of copper carbonate that reacted

   Mg + H₂SO₄ → MgSO₄ + H₂ 

• Molar ratio for Mg : H₂ is 1:1
• The mass, m, of Mg can then be determined using:
  • m = n x M_r

Source of errors

• Some hydrogen may escape from the conical flask before inserting the bung
  • This can be avoided by inserting the bung as soon as the acid is poured into the conical flask
• Some of the magnesium may not react
  • Use magnesium powder instead of ribbon  
  • Ensure an excess of acid is added
  • Swirl the conical flask to ensure the contents are sufficiently mixed and the reaction goes to completion
• Some magnesium may remain on the weighing apparatus and not be transferred to the conical flask
  • Weigh the weighing apparatus with and without and magnesium on it 
  • The difference between these gives the exact amount added to the conical flask

PAG 1.3 - Determination of the formula of magnesium oxide

• A known mass of magnesium is measured and placed in a crucible with a lid
• The magnesium is then heated to react with oxygen in the air
  • The lid of the crucible needs to be open enough to allow sufficient air in but closed enough to not lose the product
• The mass of the product is then measured

Calculations

• With the mass recordings, the original mass of magnesium is known and the mass of oxygen can be deduced
• The number of moles of magnesium and the number of moles of oxygen can then be calculated using moles =
• The molar ratio of magnesium : oxygen is then used to determine the formula of magnesium oxide,
  • For example, if the molar ratio was 2 magnesium : 3 oxygen then the formula would be Mg₂O₃ 

Sources of error

• Some magnesium oxide may escape from the crucible
  • This cannot be avoided but can be reduced by careful heating and careful placement of the crucible lid
• Some of the magnesium may not react
  • Using magnesium ribbon can be better than powder as once ignited the ribbon should continue to burn itself, while use of powder can lead to a loss a product due to the speed of the reaction as well as the potential for some of the magnesium to not react
  • If magnesium ribbon is used, it can be cleaned with sandpaper / glasspaper to remove any oxide coating already formed
• Some magnesium may remain on the weighing apparatus and not be transferred to the crucible
  • Weigh the weighing apparatus with and without magnesium on it 
  • The difference between these gives the exact amount added to the crucible