PAG 1 Moles Determination
- 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
- Measuring gas volumes
PAG 1.1 - Determination of the composition of copper(II) carbonate basic
- Set up the apparatus using a measuring cylinder sitting in water bath connected to a conical flask
- Measure a known mass of CuCO3.Cu(OH)2 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 carbon dioxide in the measuring cylinder
Equipment set up for gas collection by displacement
Examiner Tip
As an alternative a gas syringe could be set up to measure the volume of gas produced
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 Mr
- 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 Mr
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
Examiner Tip
- The relative atomic mass of magnesium cannot be completed by measuring the mass lost during the reaction
- This is because the mass lost will be relatively insignificant as it is hydrogen that will be being released
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