Answer the following questions: 1.why is the line on the cuvette always aligned

Question

Answer the following questions:

1.why is the line on the cuvette always aligned with that of the sample holder?

2. Why was hydroxylamine hydrochloride added to our solution?

3.Iron (II) reacts with water by a hydrolysis reaction . To prevent this hydrolysis, acid has been added to the standard iron solution. How would your final results change if no acid had been added to the standard iron solution?

4.Suppose a solution of Co(NO3)2 has an extinction coefficient of 5.1Lmol-cm at 505 nm. On the graph paper provided, plot a graph of A versus C (mol/L) for solutions of 0.020,0.040,0.060,0.080, and 0.100 M Co(NO3)2 in a 1 cm cell.On the same graph, plot the percent transmittance, %T, of each solution versus concentration.

5. An 8.64 ppm (1 ppm = 1 mg/L) solution of FeNCS^2+ has a transmittance of 0.295 when measured in a 1.00 cm cell at 580 nm. Calculate the extinction coefficient at this wavellength.

Explanation / Answer

In near or neutral pH conditions, Iron(II) reacts with water by a hydrolysis reaction. The product is very very easily oxidised from iron(II) to iron(III) .. by dissolved air in the solution or contact with air at the surface. [Try making some iron(II) hydoxide by adding OH- ions to Fe2+ soln in a test tube and leave for 30 min to see this]
In an acid solution the Fe2+ ion is really stable and can be kept unchanged for a good while ... long enough to do the expt over a week or so. The acid [H+] stops the complex ion containing OH from being formed.

From what I remember, the iron ion is surrounded by six ligand positions [ is hexadentate] which can be occupied by coordinated water molecules or OH- groups [from water]. This affects the redox potentials.ie ease of converting iron(II) to iron(III). In the presence of excess acid, there are no OH- group to coordinate. When iron(III) complexes form, two of these with hexadentate ligands can join to form a bridge complex .. this is why the precipitates of iron hydroxide look gelatinous. At pre-university level chemistry it is simpliied to eg Fe(OH)3. x H2O

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