Question

Page 10 Beer's Law Akuch Pyrs to shes your werk Show your work, Aplications Attach Peyes 27, what is the value of absorbance that corresponds to 35.0 %T? 28. Molar absorpt ivity is the absorptivity when concentration is in units of molarity and the gth is in centimeters. If a 1.53 x 10 M solution has an absorbance of 0.426 in a 1.00 em cell when measured at a wavelength of 254 nm, what is the molar absorptivity of the 29. A solution suspected of containing copper is analyzed. To determine the standard curve, the absorbance of five standards ranging from 0.125 mg/ml Cu to 1.25 mg/ml Cu was measured and graphed against the concentration. The slope of the linear best fit line was determined to be 0.6136 and the y-intercept was 0.0142. If three samples of the unknown solution have absorbance values of 0.438, 0.434, and 0.439 what is the concentration of the unknown? 30. A spectrophotometer will often produce values of both Absorbance and % Transmittance. Which would be the easiest for you to use to determine the concentration of a compound in soluttion? Explain. 31. Suppose that in measuring a sample of iron using an absorption spectrophotometer, you find that the absorbance is too high for the instrument to measure accurately. Based on Beer's Law, what are two options you might employ to lower the absorbance value? 32. In an Atomic Absorbtion (AA) instrument a flame is the sample holder. In the configurations below would the short, wide flame or the tall narrow flame give the highest absorbance values for the same concentration of sample? Explain why. 33. Beer's Law has some limitations. It works well for monochromatic radiation, dilute solutions, and when the absorbing compounds are not participating in a concentration- dependent equilibrium. Plot the following data using Excel and determine the highest concentration in which Beer's Law applies. Conc (mg/L) Absorbance 1.00 0.0334 5.00 0.1635 0.00 0.3284 15.00 0.4965 20.00 0.6026 30.00 0.7781 50.00 0.8652

Answer 1

Q27

A = 2 - log10 %T

A = 2-log(35)

A = 0.4559

Q28

A = e*l*C

A = absorbance of sample

e is the molar absorptivity , the typical units are 1/M-cm

l size of cuvette, typically reported in cm

c is the molar concentration, in mol per ltier or M

A = e*l*C

0.426 = e*1*1.53*10^-5

e = 0.426 /(1*1.53*10^-5)

e = 27843.13