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You are provided with 2 salts L and M. Investigate and identify the salts from the following tests.

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Introduction

Inorganic Observation Exercise - I Candidate: Lara Jane Dudley Candidate No: 5007 You are provided with 2 salts L and M. Investigate and identify the salts from the following tests. Qualitative Investigation Introduction You are provided with 2 salts L and M. Investigate and identify the salts from the following tests. 1. Observe the appearance of the two salts. OBSERVATION INFERENCE Both salts are coloured. Coloured salts suggest that a transition metal ion may be present. L : Pale lime-green crystalline solid The following transition metal ions could be present because of their characteristic colour: � Cr3+ - green � Fe2+ - pale green � Ni2+ - green Iron (II) halides are known to be pale-coloured solids and FeCl2 is pale yellow. Therefore, this compound is unlikely to be FeCl2, but could be another iron (II) halide. M: Purple crystalline solid The purple colour is suggestive of MnO4- being present. Its most common salt is KmnO4. However, Mn2+ and Co2+(aq) ions also have a characteristic pink colour. Anhydrous cobalt (II) chloride is blue, but the hydrated salt is pink. Therefore, M could be a manganese (II) or a hydrated cobalt (II) salt. 2. Make a solution of about half of your sample of L in 10 cm� of distilled water and test 2 cm� portions as follows: (a) Add aqueous sodium hydroxide until no further change is apparent. ...read more.

Middle

� SO42- could be present. 3. Make a solution of about half your sample of M in distilled water and test 2 cm� portions as follows: (a) Add aqueous sodium hydroxide until no further change is apparent. Warm the mixture gently. OBSERVATION INFERENCE Addition of NaOH (aq) immediately forms a dark blue gelatinous precipitate. There is no further reaction when excess sodium hydroxide is added. The precipitate is insoluble in this condition. The blue precipitate turned pink when the reaction mixture was left to settle. When added to am alkali, Co2+ is known to produce a blue precipitate like the one observed for the following reaction: � Co2+ + 2OH- --> Co(OH)2 Therefore it is most likely that this transition metal ion is present in the salt. An insoluble cobalt (II) hydroxide is produced. Once the cobalt (II) hydroxide has become hydrated it will turn pink in colour - a characteristic of the Co2+ ion. (b) Add an equal volume of dilute sulphuric acid followed by a few drops of aqueous potassium manganate (VII). Leave the mixture the stand. OBSERVATION INFERENCE Addition of H2SO4 (aq) does not change the appearance of the dark pink solution of M. The purple drops of aqueous potassium manganate (VII) only deepen the colour of the solution. There is no reaction. KMnO4 is a very strong oxidising agent, which suggests that M is not easily oxidised and therefore a reducing agent may not be present in the salt. ...read more.

Conclusion

A colourless gas evolved which turned damp indicator paper red. The soaked paper was initially bright yellow, but the gas evolved from L turned it green. The orangey -red colour is particular to the Fe3+ ion, therefore making it more likely that L is an iron (II) salt since the Fe2+ would have been oxidised to Fe3+during this reaction. A thermal decomposition has occurred and one of the products is an acidic gas. From the inference of stage (2d) the gas could be SO2, which forms an acid when dissolved in water. This reinforces the proposal that a sulphate could be present. SO2 gas can act as a reducing agent to yellow Cr6+, which reduces to Cr3+ - which is typically green in colour. This theory supports the observations, thus reasoning the presence of SO42- in the salt. (b) Heat solid M in a dry tube. Test the gas evolved with blue litmus paper. OBSERVATION INFERENCE The solid melted into a liquid state, which was blue in colour. The gas evolved turned blue litmus paper pink. Anhydrous cobalt (II) chloride is blue. The formation of a gas suggestion that a decomposition reaction has occurred. The gas produced was acidic and from the inference of stage (3c), this gas could be HCl, which could only be produced if hydrolysis was occurring during this decomposition. Solid L : FeSO4(NH4)2SO4 Iron (II) ammonium sulphate Solid M : CoCl2.6H2O Hydrated Cobalt (II) chloride ?? ?? ?? ?? Lara Dudley Inorganic Observation Exercise - I Spring 2002 1 ...read more.

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