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Finding the concentration of oxalic acid in a mixture with sulphuric acid

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Introduction

The redox titration will be done between potassium permanganate and ethanedioic acid. This reaction requires acid catalyst because ethanedioic acid is too weak an acid to make the solution acidic enough to react at a reasonable rate. Sulphuric acid is in the mixture and provides the acid catalyst. The ethanedioic acid in the mixture will reduce the manganate(VII) ions (MnO4-) into manganese(II) ions (Mn2+). The potassium permanganate will oxidise the oxalic acid into carbon dioxide. H2C2O4(aq) + MnO4-(aq) ? CO2(g) + Mn2+(aq) Reactants Products H2C2O4: Carbon +3 CO2: Carbon +4 MnO4-: Manganese +7 Mn2+: Manganese +2 To work out the ionic equation, balanced electron-half equations for both the potassium permanganate and the ethanedioic acid have to be worked out. Adding electrons (e-), water (H2O) and hydrogen/hydroxide ions (H+/OH-) depending on the conditions, they can be created. Manganate(VII) ions are reduced to form manganese(II) ions. MnO4- + 8H+ + 5e- ? Mn2+ + 4H2O Ethanedioic acid is oxidised to form carbon dioxide. ...read more.

Middle

= 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: * 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. * 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. ...read more.

Conclusion

= 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. ...read more.

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