Manganate(VII) ions are reduced to form manganese(II) ions.
MnO4- + 8H+ + 5e- → Mn2+ + 4H2O
Ethanedioic acid is oxidised to form carbon dioxide.
H2C2O4 → 2CO2 + 2H+ + 2e-
The two equations combine and are balanced to create the overall ionic equation.
2MnO4- + 6H+ + 5H2C2O4 → 2Mn2+ + 8H2O + 10CO2
Equipment list:
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Potassium permanganate (0.02moldm-3)
- Mixture of oxalic acid and sulphuric acid
- Conical flask
- 25ml pipette
- Pipette filler
- Funnel
- Burette
- Retort stand
- Clamp
Method:
- Use the pipette and the pipette filler to collect 25ml of the mixture of oxalic acid and sulphuric acid and put it in the conical flask.
- Use the clamp to connect the burette with the retort stand.
- Use the funnel to fill the burette with the potassium permanganate until the meniscus reaches zero.
- Place the conical flask with the mixture underneath the point of the burette.
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Using the stopcock on the burette, enter the potassium permanganate into the conical flask until there is a colour change to produce a rough estimate. Record this in a table in cm3.
- Carry out the same experiment again slowly to record a more accurate answer. Record in the table.
- Repeat step 7) until two concordant results have been collected.
Diagram:
Example of table:
The mean titre will be the volume of potassium permanganate used and the concentration of the oxalic acid can now be found. For example, if the volume of potassium permanganate used was 12.30cm3;
n=cv [KMnO4] = 0.02moldm-3; v = 0.0123dm3
n = 0.000246mol
Ratio of MnO4-:H2C2O4 is 2:5
Therefore, n(H2C2O4) = 0.000615mol
H2C2O4; c = n/v; n = 0.000615mol; v = 0.025dm3
[H2C2O4] = 0.0246moldm-3
Risk assessment:
- Potassium permanganate - Harmful; gloves should be worn. Contact with combustible materials may cause fire.
- Sulphuric acid - Corrosive; gloves must be worn.
- Goggles should be worn for eye protection.
Other points to consider:
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The concentration of the potassium permanganate is 0.02moldm-3. This is because it is the concentration it is stored in labs and can be purchased. Potassium permanganate at a high concentration is poisonous.
- The results recorded in the table will be to two decimal places.
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The pipette is more accurate than the burette with +0.05cm3 compared to +0.04cm3.
The second experiment is a gas collection. Calcium carbonate is an example of a substance which can be used in this experiment. The equation for a reaction between calcium carbonate and sulphuric acid is;
CaCO3 + H2SO4 → CaSO4 + H2O + CO2
A 100cm3 measuring cylinder will collect the carbon dioxide. To ensure the amount of gas released isn’t too much, the maximum amount of calcium carbonate added must be established.
One mole of gas occupies 24000cm3
1/240mol occupies 100cm3
Ratio of CO2:CaCO3 is 1:1
Therefore, maximum no. of moles of CaCO3 is 1/240
n x Mr = mass; n = 1/240; Mr(CaCO3) = 100
Maximum mass of CaCO3 = 0.417g to 3sf.
Equipment list:
- Calcium carbonate (mass<0.417g)
- Mixture of oxalic acid and sulphuric acid
- Tub
- Water
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100cm3 measuring cylinder x2
- Conical flask (w/rubber tubing)
- Block of wood
- Cork
- Weighing scales
- Weighing boat
Method:
- Fill the tub with water.
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Measure 25cm3 of the mixture with a measuring cylinder and put it in the conical flask with the rubber tubing.
- Fill the other measuring cylinder with water from the tub and invert it in the tub, with aim to avoid any air entering it.
- Place the conical flask on a block of wood.
- The other end of the rubber tubing should be placed underneath the inverted measuring cylinder.
- Using the weighing scales, measure any mass of calcium carbonate below 0.416g via a weighing boat.
- Pour the calcium carbonate from the weighing boat into the conical flask and immediately put the cork on the conical flask.
- Record the amount of gas released in the measuring cylinder.
Diagram:
The concentration of the acid can now be worked out. If, for example, 88cm3 of carbon dioxide was released;
One mole of gas occupies 24000cm3
(1/24000)x88 = n(CO2) = 0.00367mol
Ratio of CO2:H2SO4 is 1:1
Therefore, n(H2SO4) = 0.00367mol
c=n/v; n = 0.00367; v = 0.025dm3
[H2SO4] = 0.147moldm-3
Risk assessment:
- Sulphuric acid - Corrosive; gloves must be worn.
- Goggles should be worn for eye protection.
Other points to consider:
-
We can see in the example that if 88cm3 is released, the concentration is 0.147moldm-3. This is not approximately 0.2moldm-3. If 100cm3 of CO2 gas was released, [H2SO4] would be 0.167moldm-3. But if [H2SO4] was nearer the approximated value, not all of the acidic molecules would have reacted. To solve this problem, a 250cm3 measuring cylinder could be used. This can collect more gas and also leads to a larger maximum amount of calcium carbonate which can be used, which increases the chance of all the acidic molecules reacting. However, with this change, the accuracy decreases. The 250cm3 measuring cylinder has an accuracy of +0.5cm3 compared to the 100cm3 measuring cylinder with +0.25cm3.
- It is difficult to put the cork on the conical flask directly after entering the calcium carbonate into it. Also, carbon dioxide is soluble in water. Both decrease the chance of 100% yield.
Bibliography;
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