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Determination of the formula of Hydrated Iron (II) Sulphate crystals

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Introduction

Determination of the formula of Hydrated Iron (II) Sulphate crystals Analysis Method one. In order to find out the ratio of H2O to FeSO4, the mass of water needs to be calculated, which is done by weighing the iron sulphate before and after the heating. This will tell us the weight that was lost and thus the weight of the water. Grams (g) Empty crucible 12.01 Crucible and Iron (II) sulphate crystals 13.48 Mass of Iron (II) sulphate crystals (2-1) (13.48-12.01) 1.47 Crucible and Anhydrous Iron (II) sulphate 12.82 Mass of Anhydrous Iron (II) sulphate (5-1) (12.82-12.01) 0.81 Mass of water (13.48-12.82) 0.66 Repeat 7. Crucible and Anhydrous Iron (II) sulphate (after re-heating) 12.80 8. Mass of Anhydrous Iron (II) sulphate (after re-heating) (7-1) (12.80-12.01) 0.79 9. Mass of water lost during repeat (4-7) (12.82-12.80) 0.02 10. Mass of water lost altogether (2-7) (13.48-12.80) 0.68 Finding the molar ratio between FeSO4 and H2O requires finding the empirical formula using their masses. To find the number of moles of each of the compounds, the equation moles= mass/molar mass is used. Finding the Atomic masses of FeSO4 and H2O: Atomic Masses Fe= 55.8 S=32.1 O4= 16x4 H2=1x2 O=16 Molar mass of FeSO4= 55.8 + 32.1 + (16x4) = 151.9 Molar mass of H2O = (1x2) + 16 =18 FeSO4 H2O Mass 0.79g 0.68 Moles 0.79/151.9= 0.005 0.68/18=0.038 Finding the Ratio: 0.005/0.005 0.038/0.005 =1 =7.4 Ratio of FeSO4 to H2O is: 1:7 Therefore, the formula for Iron(II) Sulphate in this experiment is; FeSO4.7H2O Thus, the overall equation for the reaction was: FeSO4.7H2O(s) -> FeSO4(s) + 7H2O(l) Observations and comments As heated, the pale green crystals became whiter and joined together to form one block, which became grey around the sides of the crucible. This was because this part of the FeSO4 closest to the heat and so was beginning to be decomposed. When the 'anhydrous' iron sulphate taken out of the crucible after procedure completed; red on the underneath of the iron sulphate. ...read more.

Middle

Another procedural uncertainty is the possibility that the anhydrous iron sulphate combined with moisture in the air following the heating process and so, ultimately was not completely dehydrated thus weighing more than the anhydrous iron sulphate would have. All of the above procedural uncertainties have a detrimental affect on the accuracy of the results. The most significant of the procedural uncertainties however is the uneven heating that occurred and resulted in the part decomposition of the iron (II) sulphate. This is the most important uncertainty because it meant that whilst some of the iron (II) sulphate remained hydrated, other parts decomposed, which meant that the procedure carried out was very inaccurate, failing to find the mass of just anhydrous iron sulphate as should have been done. In order to alleviate this uncertainty a number of improvements can be made. Firstly, a shallower dish should be used. If this was used together with a powdered form of the iron (II) sulphate, this would ensure that the heat reached all parts of the iron (II) sulphate equally. In addition, to prevent any chance of the iron (II) sulphate combining with any substance that may have already been on the shallow dish including moisture; the dish should be washed and dried by heating it and then cooled in a desiccator so that it doesn't pick up any moisture from the air. Then, after heating the iron (II) sulphate; place the shallow dish with the anhydrous iron (II) sulphate back into a desiccator to cool; this would ensure that the anhydrous form did not combine with any moisture in the air on cooling. Before placing the heated evaporating dish in to the desiccator, allow it to cool for one minute and leave the desiccator lid open slightly for 30 seconds once it the hot dish has been placed in side. This is done in order to ensure that as the heated air cools, a partial vacuum is not formed. ...read more.

Conclusion

This makes the method very unreliable as there are no other results to compare to, to ensure that the mass of water calculated is not simply an anomaly. Moreover, finding the average between a number of results increases the accuracy of the overall result but you can not do this with method one. Additionally, as the procedure was not repeated until the iron sulphate reached a constant mass, it is quite likely that not all of the water was driven off; making the weight of water calculated not entirely accurate. Conversely, in method two, 4 titrations were completed and an average was found from 3. Not only does this method allow you to identify an anomaly but it allows you to find an average between a number of titres, which means you are more likely to get closer to the actual answer. There is the procedural uncertainty with method two surrounding the loss of Fe2+ ions due to transferring them but since the mixing container and weighing boat were rinsed with de-ionised water, the loss of ions should have been at a minimum. However, as the procedure in method one stands, there are a number of procedural uncertainties including the deepness of the crucible and the iron (II)sulphate crystals as well as not knowing how intense the heat should be. All of these issues can lead to the uneven heating of the iron (II) sulphate and ultimately, the part decomposition of the iron (II) sulphate. This means that when weighing what should be only anhydrous iron (II) sulphate, you are not. If this problem can arise when I was conduction the experiment, this means it could arise again suggesting that the method has some significant flaws. Therefore, comparing the two methods; I believe that method two is the most accurate. This is for the reasons stated above and although method two only repeats the titration three times; method one only obtains one mass of water. Moreover, the procedural uncertainties concerning method one are likely to have a more significant affect on the accuracy of the results than the procedural uncertainty of method two. ...read more.

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