Old Exam III

Chemistry 422 Exam III	Name:
December 4, 1998		Please print

Instructions: Read the whole exam before starting to work on individual problems; work those that you find the easiest first. Budget you time!

a. What are shims in a NMR spectrometer, and what is their purpose?

Why is there no observed splitting between carbons in ¹³C spectra?

Draw a diagram of the major components of a NMR.

Calculate the relative number of protons (g = 2.68x10⁸T- ¹s- ¹) in the higher and lower magnetic states when a sample is placed in a 7.00-T field at 20.0°C.

Solution:

a-c see notes

d. Use the simplified form of the Boltzmann equation:

a. Draw a diagram of the major components of the electron impact ionization part of a mass spectrometer.

b. Explain why a vacuum is required in mass spectrometry.

The time of flight of a molecule having a molecular mass of 50.0 g/mol in a mass spectrometer after being accelerated with 2000 V was 5.00 ms. What would the time be for a molecule with a molecular mass of 75.0 g/mol after being accelerated with 2500 V?

Solution:

3. Determine the K_sp of Ag₂CrO₄(s) at 298K using the information given below.

Ag⁺(aq)+ e-	« Ag(s)	E^o = +0.799 V
Ag₂CrO₄(s) + 2 e-	« 2Ag(s) +	E^o = +0.446 V

Solution:

2Ag(s)	« 2Ag⁺(aq)+ 2e-	E^o = - 0.799 V
Ag₂CrO₄(s) + 2 e-	« 2Ag(s) +	E^o = +0.446 V
Ag₂CrO₄(s)	« 2Ag⁺(aq) +

4. Determine the length of time (s) it would take to reduce the concentration of iron from 0.0200 M to 0.00500 M. Use graph below (left).

Solution:

5. a.Determine the number of electrons involved in the reduction from the polarographic wave shown in the graph on the right. Process is everything here! No points will be given for a correct answer without showing the work!

b. Draw a diagram of the major components of a polarograph and explain the principles of its operation.

Solution: i_d = 5.0, E_1/2 = - 0.40 V; E₁ = - 0.375, i₁ = 0.62 Þ n = 2.

6. Determine the number of theoretical plates and the height equivalent of a theoretical plate using the graph to the right and given that the column was 30.0 cm long.

Solution: Use the normal equation for theoretical plates. The width of the peak is 0.5 and gives a total of 11, 000 theoretical plates with H = 2.78x10- ³ cm