The absorbance at 470 nm of the Fe(Il)-phenanthroline complex was measured for s

Question

The absorbance at 470 nm of the Fe(Il)-phenanthroline complex was measured for several different Fe concentrations in order to create a calibration curve. The absorbance at each Fe2+ concentration was measured five times. The readings, already corrected for the blank solutions, are shown below Map a Fe21 (M) A 75.9 53.5 37.7 15.2 A3 A4 0.582 0.591 0.587 0.5830.588.5862.0585919.6 0.412 0.411 0.421 0.414 0.418 .41520050 62.8 0.2640.266 0.2690.2580.26626460063 62.0 sum182.3 average45.575 1.3671.381 1.385 1.3701.383 1.3771242 1967 0.34180.345 0.346 0.343 0.346 3443 Incorrect For part (a), use a graphing calculator or the SLOPE and INTERCEPT functions of Excel to calculate the slope and y-intercept. Use the averages of the replicate absorbance measurements (see below) in the calculations Number (a) Use the averages of the replicate absorbance measurements to determine the slope and y-intercept of the best-fit straight line m-0.0079 Number 110.0173 (b) Predict the Fe concentration in an unknown sample that has an absorbance of 0.133 Fe21 (IM) Aav 75.9 53.5 37.7 15.2 0.5862 0.4152 0.2646 0.1112 Number 19.03 (c) What is the uncertainty associated with the predicted Fe2' concentration? Number 0.018 Previous Give Up & View SolutionTry Again Next Exit

Explanation / Answer

For calculating the uncertainty in predicted concentration we need to calculate the uncertainty in the absorbance

Aavg = (A1 + A2 + A3 + A4 + A5 )/5 = 0.586

Therefore the uncertainty in the average concentration can be calculated by

u Aavg = [ (u1)2 + (u2)2 + (u3 )2 + (u4)2 + (u5 )2 ]0.5

where u1 = uncertainty in the absorbance value A1 = 0.001 ( equal to the least count of the measuring device)

ui = 0.001 for i = 1,2,3,4,5

Therefore calculate the value of  u Aavg we get

u Aavg = 2.236 * 10-3

Now uncertainty in concentration is given by

uc /C = uA /A

From the calibration curve, we know that the concentration is 19.03 at absorbance equal to 0.133

So the uncertainty in the predicted value of concentration is

uc = C * uA / A = 19.03* 2.236*10-3 / 0.133 = 0.309 uM

I hope it helps!!

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