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Deduce the concentration of a saturated solution of chlorine

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Aims: Deduce the concentration of a saturated solution of chlorine In order to determine the concentration of aqueous chlorine one must firstly reduce the chlorine to its ions, using a reducing agent, in this case Fe(II). I will be using FeCl2 powder for a source of Fe2+. 2FeCl2 (2) + Cl2(aq) --> 2FeCl3 (aq) [ EQUATION 1] 2Fe2+ (aq) + Cl2 (aq) --> 2Fe3+ (aq) + 2Cl-(aq) 2 : 1 ratio Now if one can calculate the amount of reducing agent left over, it will be possible to calculate the initial Cl2 solution concentration. It is possible to do this by titration of the Fe2+ with Potassium Manganate. [ EQUATION 2] MnO4- (aq) + 5Fe2+(aq) +8H+(aq) --> Mn2+ (aq) + 5Fe3+(aq) + 4H2O(l) 1 : 5 ratio Acidic conditions are necessary, because in neutral or alkaline conditions Potassium Manganate is reduced no farther than Mn+4 and a source of H+ is also needed. Sulphuric acid is a good source of H+, and the SO4-2 ions are not reactive. Hydrochloric acid would react with the KMnO4, and chlorine gas would be evolved. Nitric acid is not suitable as it can become involved in redox processes where the NO3- ion is reduced. The end-point of the titration is detected by the first persisting pale pink colour change from purple. ...read more.


[burette reading] * Conc of MnO4- = 0. 00118 mol / 0.020dm-3 = 0.059 mol dm-3 Acid required * Using equation 2 we can see that the reaction requires 8x the number of moles of MnO4 * Moles of MnO4- = 0. 00118 mol * Moles of Acid required (at least) = 0.00944 mol * 0.025 dm3 of 0.5 mol dm-3 H2SO4 = 0.0125 mol This gives a safe excess of acid to use in the titration reaction From these calculations you can see that one should use at least 0.125g of FeCl2 reducing agent and MnO4- of concentration approximately 0.06 mol dm-3 for the titration. Method Equipment: 50 cm3 Burette 50 cm3 Volumetric Pipette Pipette Filler 250 cm3 Conical Flask Clamp Stand White filter paper Glass rod Balancing scales Weighing boat 1. Use a volumetric pipette and pipette filler to deliver 50.0 cm3 of the chlorine solution into a 250 cm3 conical flask. Using the balancing scales and a weighing boat measure out approximately 2.0 g of FeCl2 solid. Record the mass of the weighing boat then add FeCl2 until the mass reads equal to the initial weight of the weighing boat + 2.0 g. Record the weight of FeCl2 - total weight of boat and FeCl2 - initial boat weight. ...read more.


moles FeCl2 = no. moles Fe2+ = 0.00591 mol * 0.00591 moles of FeCl2 in grams = 0.00591 x 126.75 = 0.749 g Fe2+ * If there is 0.749g FeCl2 left over from the 2g, then 1.251 g Fe2+ must been reduced * No Moles of Fe2+ reduced = No Moles of FeCl2 reduced * No Moles of Fe2+ reduced = 1.251/126.75 = 0.00988 * No Moles of Cl2 = 0.5 x No Moles Fe2+ in accordance with equation 1 * No Moles of Cl2 = 0.00494 * Concentration of Cl2 solution = mol/volume(dm-3) = 0.0988 mol dm-3 * 70.90 x 0.0988 = 7.00g * Concentration of Cl2 solution = 7.00 g dm-3 Risk Assessment When dealing with any chemicals a lab coat and goggles should be worn at all times. Potassium Permanganate in its solid form is a very strong oxidizer and if it comes into contact with other material may cause a fire. However as we will only be using a dilute aqueous solution this hazard will be minimal. Even in aqueous state it may cause severe digestive tract irritation or eye irritation which is why it is essential to wear goggles. Sulphuric acid is corrosive and an irritant, even though only weak concentrations are being used eye protection and lab coats should be used throughout. When dealing with glassware extra precautions should be taken, do not use glassware above shoulder level; always fill the burette from below shoulder height. CHRISTOPHER TRANTER ...read more.

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