Analysis and identification of two unknown polyiodide anions from two different products

Determination of iodide anions calculations

Additional data values were used

1. Assuming Product A contains the  anion:

Molar mass of Agl = 234.77 g/mol

Molar mass of  = 454.8582 g/mol

Moles of :

 mol
Expected Agl to precipitate:

 mol or 0.2787 g

                                                                                                                             

Actual Agl precipitated (value obtained from table):  mol

2. Assuming Product B contains the  anion:

Molar mass of Agl = 234.77 g/mol

Molar mass of  = 454.8582 g/mol

Moles of :

 mol  

Expected Agl to precipitate:

 mol or 0.2168                                                                                                                          

Actual Agl precipitated (value obtained from table):  mol

3. Assuming Product A contains the  anion:

Molar mass of Agl = 234.77 g/mol

Molar mass of  = 708.6671 g/mol

Moles of :

 mol
Expected Agl to precipitate:

 mol or 0.2982 g

                                                                                                                             

Actual Agl precipitated (value obtained from table):  mol

4. Assuming Product B contains the  anion:

Molar mass of Agl = 234.77 g/mol

Molar mass of  = 708.6671 g/mol

Moles of :

 mol
Expected Agl to precipitate:

 mol or 0.2319 g

                                                                                                                             

Actual Agl precipitated (value obtained from table):  mol

The difference between the precipitated mass and the expected precipitated mass when assuming anion present:

Mass difference between Product A precipitate and expected precipitate:

                                                                                                           g

Mass difference between Product B precipitate and expected precipitate:

                                                                                                           g

Because the smallest difference in mass is obtained when taking Product A in consideration rather than Product B, one can reasonably assume that the anion is present in Product A.

The difference between the precipitated mass and the expected precipitated mass when assuming anion present:

Mass difference between Product A precipitate and expected precipitate:

                                                                                                           g

Mass difference between Product B precipitate and expected precipitate:

                                                                                                           g

Because the smallest difference in mass is obtained when taking Product B in consideration rather than Product A, one can reasonably assume that the anion is present in Product B.

Table 5: Molar mass of each compound

Percentage Yield

Product A

 is the limiting reagent

 mol

     1:1 mol ratio for  to , so mol of   is theoretically needed

  mol (actual)

1 mol  : 1 mol

 mol  used (actual)

 The theoretical mass of required =  mol  g/mol

                                                                                    =   g

Product B

 is the limiting reagent

 mol

     1:2 mol ratio for  to , so mol of   is theoretically needed

  mol (actual)

2 mol  : 1 mol , so

 mol  used (actual)

 The theoretical mass of required =  mol  g/mol

                                                                                 =   g

Discussion

Although polyiodide salts are highly complex structured compounds, they can very easily be made by mixing iodine with iodine salts in different ratios. The structure of the polyiodide is completely dependent on the specific compound. (Michael Groessl, 2011)

This was done by analysing the moles/mass present in both unknown Products A and B. By using two different amounts of tetramethyl ammonium iodide for the two products, A and B, two different coloured crystal compounds were obtained.

Product A was observed to form tiny purple crystals and it was determined that the  anion was present. Similarly, Product B formed long shard like crystals that were a metallic green colour. It was determined that the green crystals contained the  anion.

The percentage yield of   and  were calculated to be 78.6% and 76% respectively. These yields differed from those obtained by other students. This might be due to the varying amounts of liquid that was not drained off by air suction and thus remained with the crystal. The yields calculated did vary quite a bit from the literate value yields, and this could be due to the difference in mass of the final product as well.

Taking all factors into consideration, one can still say with certainty that the experiment performed as well as the results obtained were indeed valid and allows one to assume that Product A contained the  anion and Product B the anion.

Questions

1. Why is a large cation required in order to obtain stable complexes of  ?

Because both  and  are large anions, a cation that has the same size and magnitude is required in order to balance the charges. These anions and cations can only become stable when they are similar in size.

2. Draw Lewis structures for .

3. Use VSEPR theory to predict the shapes of .

The  anion will have a linear shape with a trigonal bipyramidal electron geometry. Where as the  anion will have a octahedral molecular shape and a bent shape electron geometry.

Conclusion

Two unknown products, A and B, were obtained and analysed in order to determine the unknown polyiodide anions present in each product. The appearance of Product A was characterised by tiny splinter like purple crystals, where as Product B formed long shard like crystals with a metallic green colour. The percentage yields of  and  were 78.6% and 76% respectively, which deviated quite a bit from the literate values that were 39.78% and 39.72% respectively, which might be due to the liquid retained by the crystal compounds and thus not removed by air suction while performing the experiment. But the values obtained are not unrealistic and thus one can assume that the procedure followed, and data collected are valid.

References

Michael Groessl, †. Z. (2011). Mass Spectrometric and Theoretical Study of Polyiodides: The. Inorganic Chemistry, 1.