Question

When measuring the volumes of Fe(NO3)3 and NaSCN solutions in this experiment, the student mistakenly used a graduated cylinder instead of volumtetric pipets, After collecting all of the data, the student realized he'd used the wrong piece of equipment, but he didn't redo the experiment. Also, he later realized that he had consistenly misread the graduated cylinder and had thus transferred volumes that were actually 5% lower that the recorded volumes. Incoportate these measurement errors into the data for one of your equilibrium solutions, and recompute Keq for that solution. Determine whether the student's measurements errors would cause each of the following, as recalculated by you, to be higher than, lower than, or identical to the value you originally determinded. Briefly explain.

1) the calculated SCN- ion concentration in the standard solution

2) the slope of the Beer's Law plot

3) the calculated equilibrium solution concentrations

4) calculated Keq

Answer 1

I am working to answer your question. If you have a description of the experiment it is a good idea to let me know it.

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Only to have a reference I will assume a procedure like this one:

http://www.jackson.k12.ga.us/teachers/rbryan/AP_Chemistry_Online/LabReport%20ExampleKeq.pdf

Fe³⁺ + SCN⁻   = FeSCN²⁺

The 5% error was a systematic error and can be corrected.

a).

A standard solution of SCN^- was prepared by transfering a volume V (in fact 0.95V or 95% V) from the stock solution into a volumetric flask. Fe³⁺ was added in high excess to allow SCN^- to completely react, thus the error in measuring volume for Fe³⁺ is not relevant.

The procedure was repeated with a series of different V volumes (C concentrations of SCN^-). The actual concentrations C are actually 95% of the calculated values. A correction is possible (multiply the calculated C values by 0.95).

b.)

A series of absorbancies was recorded. A plot A vs. C is an equation:

A = sC + i      (s= slope, i= intercept).

s = (A-i)/C . If C (of SCN^- and also FeSCN²⁺) is in fact lower, the absorbancies are (about 5%) lower and the slope is lower.

But if C are corrected at a) , the calibration curve is correct (unaffected).

c)

A new series of mixtures (Fe³⁺ + SCN⁻ ) was prepared with concentrations more close one to each other, e.g., [Fe³⁺]/[SCN^-] in ratios 5:1 , 4:1, 3:1, 2:1, 1:1.

Each concentration for [Fe³⁺] and [SCN^-] was in fact lower (95%), but the ratios were not affected. Using the wrong Beer’s Law plot

C = (A-i)/s   with a lower s, the calculated C are higher (the actual [FeESCN²⁺ is only 95%).

To correct the data, use the corrected slope from b).

d)

K_correct = [FeSCN²⁺]/([Fe³⁺][SCN^-])

The initial calculation was made with

1.05[FeSCN²⁺]/(0.95[Fe³⁺]·0.95[SCN^-]) = K_wrong= 1.16K_correct

Correct K_wrong by 1.16 or use the corrected data from a,b,c for a re-calculation.