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Finding the empirical formula of aluminium chloride.

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A-Level Chemistry Coursework Investigation:- Finding the Empirical Formula Of Aluminium Chloride By Yasir Al-Wakeel Introduction: The purpose of this experiment is to find the empirical formula of a sample of anhydrous aluminium chloride by titrimetric analysis. The empirical formula of a compound, as opposed to the molecular compound, is the simplest formula which represents its composition. This shows the elements present and the ratio of these elements. The molecular formula, on the other hand, is a simple multiple of the empirical formula and shows the actual number of atoms of the different elements in one molecule of the compound. Thus ethene, a homologue of the alkenes, has a molecular formula of C2H4 , whilst the empirical formula of ethene is CH2. To find an empirical formula the ratio of the amounts (in moles) of the elements is required, which in this particular experiment is to be found by means of titration. Titration is a type of volumetric analysis used to find the amount of a substance in solution. A 'standard' solution, or solution of known concentration, is reacted with a certain volume of the solution under investigation and the volume of the standard required to complete the reaction is recorded. In some cases the end-point of the reaction is self-indicating, yet in most cases, including this experiment, an indicator is required. Indicators change colour to show the reaction is complete. The presence of colour and colour change are usually due to the presence of transition metals, as ions, in the indicator. Transition elements are those in the d-block of the periodic table and can be defined as an element with at least one ion with a partially filled d-subshell The 'phenomenon' of colour is related to the incompletely filled d-orbitals in the transition metal ion. In short, light photons falling on transition element compounds interacts with the d-electrons, exciting them and causing electron transitions. ...read more.


However, their will still be a little of the solution remaining at the bottom of the pipette, yet this should not be 'blown' out as this residue is taken into account when the instrument is calibrated. 5. Small portions of calcium carbonate are added to the conical flask until there is no more effervescence and a small amount of unreacted powder remains. 6. Ten drops of potassium chromate indicator, which is yellow in colour, is then added to the solution 7. A burette, of 50cm3 in capacity, is then clamped to a stand, whilst making sure that the burette is vertical. 0.05 molar solution of silver nitrate is then acquired. Silver nitrate is poisonous and can stain the skin, therefore necessary safety precautions must be taken. The burette is first rinsed with the silver nitrate solution, for maximum accuracy, and is then filled up to the calibration mark using a funnel to prevent spillage. Any air bubbles present must be removed as these help accumulate our experimental error. 8. The silver nitrate solution is then titrated into the conical flask. This is done by squeezing clippers at the bottom of the burette to allow the silver nitrate to flow. Care is taken to prevent the silver nitrate from going on to the sides of the conical flask and making sure it goes directly into the chloride solution. The conical flask is gently, but continuously swirled throughout the titration. At first, a white precipitate of silver chloride appears, but the titration is continued, with shaking, until the first permanent faint reddish tinge of silver chromate is seen. The first titration is a trial run and gives us an indication of the volume of titre required. The titration is then repeated with greater accuracy until, ideally, when consecutive results agree to within 0.1cm3. Results and Calculations Results: Mass of weighing bottle before, m1 +0.0005 4.688g Mass of weighing bottle after, m2 +0.0005 5.688g Mass of Aluminium Chloride, m1-m2 +0.001 1.000g N.B.- The error is taken as the maximum error-absolute error-of the instrument. ...read more.


In order to limit this, even though of very small significance, the mass of the bottle before and after removing the aluminium chloride should have been noted and hence we would have known the exact mass of aluminium chloride added to the jar. Some of the aluminium chloride may have also reacted with moisture in the air, however, as exposure to the air was kept to a minimum and no hydrogen chloride gas was seen evolved, this too is of negligible importance. Measuring out a 100cm3 of water in the volumetric flask and pipetting this into the conical flask is likely to have been of high accuracy. Yet the source of error that is most likely to have contributed mostly to the 2.1% error attained, was the measurement of the titre. The burette was accurate to the nearest millimetre, and hence the error in the mean is: 0.1 x 100 = 0.22 % 44.75 Although this is very accurate, there are additional errors that were involved in this measurement, notably judging the end-point of the titration. A little of the titre ended-up on the sides of the conical flask and the change from a white precipitate to a permanent faint reddish tinge of silver chromate is indeed hard to judge perfectly, as white to faint red is not very distinct and in addition, the white was already changing to faint-red, but this was not permanent. However, the fact that this was repeated, reduced the likely error. Using a burette accurate to a tenth of a millimetre, if there is one for the volume used, would be a useful improvement to the method. In conclusion the results attained, given that there were no anomalous errors and the experimental error was very low, were very satisfactory, and have verified, to a reasonable extent, the theory in the introduction stating that the empirical formula of the aluminium chloride is AlCl3. Another related experiment may be designed to determine whether or not the molecular formula of aluminium chloride is Al2Cl3. ...read more.

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