Chem141

CHEM 141 Homework/ Problem Set 7

Please work through the following questions and find the appropriate multiple-choice and True/false answers. They are mostly based on Chapters 18-22 of Harris' text-book. Maximum score 65pts

Please input your answers via the LMES2 server system after logging-on and going to Quizzes and Tests. One change of submission will be allowed.

Submission deadline 12 midnight, Thursday 10 December

Q1. Absorbance and Transmittance (6pts)

Which of the statements a to d) are false ?

a) If the concentration of a sample is doubled the transmittance is decreased by a factor of four.

b) If the absorbance of a sample is 0.30 the transmittance is ca. 50%.

c) If the absorption coefficient ε of a compound at λ2 is 2x that at λ1 then the measured absorbance of a solution of the compound at λ1 must be half of that at λ2.

d) If monochromatic light passed sequentially through solutions in two cells each with an absorbance of 1.0 then the total fraction of the incident light passing through would be ca 10%.

e) All of a) to d) are correct.

Incorrect statements: a) c) and d)

c) is correct only if absorbtion of solvent can be neglected. Since λ1 and 2 are unknown it certainly doesn’t hold in general and Q2 should be a pretty big hint in case you didn’t attend class.

Q2. Absorption Coefficient (5pts)

Hexane is used as a solvent for uv-spectroscopy since it has a low absorption and far uv cut-off. The specification sheet for spectrophotometric grade hexane from Aldrich indicates that the absorbance at 240 nm is 0.020. Given the Fw of hexane is 86.18 and the density 0.659 g cm-3, what is the molar absorptivity (in M-1 cm-1) at 240nm if the absorbance value is based on a 1.000cm path cell ? (give answer with 3 sig figs)

a) 2.60 x 10-4 b) 2.62 x 10-3 c) 6.03 x 10-3 d) 0.152 e) 382

f) None of the above g) Cannot be determined

Correct answer b) [0.020/ 1.000 x (659/86.18)] = 0.020/ 7.647 = 2.615 x 10-3

Q3....