Work out concentration of the ethanedioic acid
Now that I Know the volume of KMnO4 that was required for the redox reaction, I can work out the number of moles of KMnO4
Concentration x Volume = Moles, therefore 0.05 x average titre=moles of KMnO4
From the equation I know that the Molar ratio between KMnO4 and H2C2O4 is:
5C2O42- 2MnO4- , therefore 5:2 ratio. So if I divide moles of KMnO4 by 2 and then multiply by 5 this gives the moles of H2C2O4. And finally to get the concentration of H2C2O4 simply do moles/volume, so moles of H2C2O4 / 0.025dm3 = concentration of H2C2O4
Experiment 2
I can use a gas collection to find the overall concentration of the acid mixture, and as I already have the concentration of H2C2O4 , I can simply minus that from the total concentration to get the concentration of the H2SO4.
Both strong and weak acids will react with a metal, therefore I will react the acid mixture with magnesium metal.
Quantities of chemicals to use
As I have been told that the concentrations are approximately 0.1 mol dm-3 H2C2O4 and 0.2 mol dm-3 H2SO4, I can say that the total concentration is approximately 0.1+0.2=0.3 mol dm-3. I am going to use 25cm3 of the acid mixture, therefore total moles of acid mixture is 0.3 x 0.025 = 0.0075 moles
H2SO4 + Mg ➔ MgSO4 + H2 and H2C2O4 + Mg ➔ MgC2O4 + H2
I need these in one equation so I can get the molar ratios, therefore I formed this ionic equation. 4H+ + 2Mg ➔ 2Mg2+ + 2H2
The molar ratio between the acid mixture hydrogen is 4:2(2:1), therefore (0.0075/2)x24000=90cm3, this is less than 250cm3, so will fit into the measuring cylinder. I must ensure the Magnesium is in excess. So if I use approximately 0.1grams, moles of Mg = 0.1/24.3=0.0042moles but 1:2 ratio so multiply by 2 to get 0.0084moles, therefore there is more moles Mg than acid so it is in excess.
Method:
- Set up the apparatus as shown in the diagram and fill the bowl with water until approximately 1/3 full.
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Using a graduated Pipette measure out 25cm3 of the acid mixture and place this into a 250cm3 conical flask.
- Place weighing boat on the scale and press tear button, then add 0.1 grams Magnesium.
- Place the magnesium metal on the split flask and then release it ensuring the bung is properly fitted to the flask to ensure that no gas can escape.
- Ensure the upside down measuring cylinder is on top of the end of the delivery tube so that it collects all the gas.
- Allow gas collection to take place, however you may need to leave it several hours as there is also a weak acid involved meaning it will dissociate slowly. However to ensure it goes to completion wait until there is no more fizzing and no more gas appearing in measuring cylinder.
- Once finished record the volume of gas collected and use this to work out moles of gas produced.
Diagram:
Safety
Ethanedioic acid is a highly poisonous carboxylic acid. It is corrosive and may cause burns. If it comes to contact with skin rinse with plenty of water immediately. Wear a lab coat to prevent exposure to skin.
Specimen calculation
If 22cm3 of KMnO4 was needed for the redox titration, then 0.022 x 0.05= 1.1x10-3 moles of KMnO4 .
Using 5:2 ratio of 5C2O42- 2MnO4- moles of H2C2O4 is (1.1x10-3 /2) x5=2.75x10-3 moles, and finally to get concentration 2.75x10-3 /0.025=0.11 mol dm-3. Which is almost 0.1mol dm-3 as suggested the concentration was approximately.
The above was all from experiment 1, and now to get the concentration of the Sulphuric acid was experiment 2.
If the volume of gas collected was 80cm3, then moles of Hydrogen would be (80/1000)/24=0.0033moles, however 4:2 ratio with acid so multiply by 2 to get 0.0066moles, and as I used 25cm3 acid, concentration is 0.0066/(25/1000)=0.27mol dm-3. To get the H2SO4 concentration minus the H2C2O4 concentration (0.11), 0.27-0.11=0.16mol dm-3, this is close to 0.20mol dm-3
