Question

In a hurry to complete the experiment, Joseph failed to calibrate the spectrophotometer. As a result, all absorbance values for the standard solutions that are measured and recorded are too high. How will this affect the following for the Test Solutions in Parts B and C? a: will the equilibrium concentrations of FeNCS2+ be too high, too low, or unaffected? Explain b: Will the equilibrium concentrations of Fe3+ be too high, too low, or unaffected? Explain. c: Will the calculated equilibrium constants be too high, too low, or unaffected? Explain

Answer 1

a) The uncaliberated spectrophotometer is showing too high values of absorbance of test solutions. So, the absorbanance value for FeNCS2+ will also be too high. The concentration of FeNCS2+ will be measured from absorbance using Beers law (According to Beer law, Concentration is directly proportional to absorbance). Therefore, the equilibrium concentrations of FeNCS2+ will be too high.

b)                   Fe³⁺ + NCS^- FeNCS²⁺

initial conc.                                                                 A           B              0

equilibrium conc.                                                       A-x         B-x            x

The concentration of FeNCS2+ (x) will be calculated from spectrophotometric data and will be too high (as told above). Using this we will calculate the equilibrium concentration of Fe3+ as above (A-x). Since a too high value will be subracted from A equilibrium conc. of Fe3+ will be too low.

c) As we all know that equilibrium constant only depends on temperature therefore it will be unaffected by the change in concentrations due to incorrect spectrophotometric measurements. We can also see it this way that the increased conc. of FeNCS2+ will be componsated by the decreased conc. of reactants and overall ratio will be unaffected.