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KKEK2233: PHYSICAL AND ANALYTICAL CHEMISTRY II

Mid Semester Test 2

Time: 1 hour (TOTAL= 80 marks)

 Answer ALL questions.

1. The formula for a cosmetic cream calls for 5% of an emulsifier blend consisting of Span

60 (HLB=4.7) and Tween 20 (HLB=16.7). If the required HLB of the oil phase is 14.0, how

many grams of each emulsifier should be used in preparing 500 grams of the cream?

(20 marks)

By allegation method

14Span60: 4.7 2.7

Tween20: 16.7 9.3

Relative Amounts

Tween20 : Span60

9.3 : 2.7 (9.3+2.7 = 12)

9.3/12 x 100 = 77.5% : 2.7/12 x 100 = 22.5%

Since 5% of emulsifying blend is used in the preparation of 500 grams of cream, therefore,

5/100 x 500 grams = 25 grams of mixture should be used in the preparation of 500 grams of

cream.

Span60 = 25 grams x 22.5/100 = 5.625 grams.

Tween20 = 25 grams x 77.5/100 = 19.375 grams

So, 5.625 grams of Span60 and 19.375 grams of Tween20 should be used in the preparation of

500 grams of cream.

2. What is critical micelle concentration (CMC)? Provide brief explanation.

(10 marks)

Notes

3. What is the main role of auxiliary electrode in voltammetry technique?

(10 marks)

Notes

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4. Ethanol and methanol are separated in capillary GC column with retention times of 370

and 385 s, respectively, and base widths (wb) of 16.0 and 17.0 s. An unretained air peak

occurs at 10.0s. Calculate the separation factor and the resolution.

(20 marks)

= ( ) = .   

=  − − = . 

 =  −   = . 

 =  −

  = . 

 =  + .   = .  

 =  √ . (.−.   ) (   . . + ) = . 

5. Predict the elution order of n-hexane and n-hexanol on the following stationary phases.

Briefly explain your answers.

(Bonus: 20 marks) No need to answer, but if answer will be bonus marks

(i) Reverse phase liquid chromatography

Elut ion o rder hexanol, hexane: order of decreasing polari ty

(ii) Normal phase chromatography

Elut ion o rder hexane, hexanol: order of in creasing polari ty

6. The following data were obtained for four compounds separated on a 20 m capillary

column.

compound tr   w

min min

 A 8.04 0.15

B 8.26 0.15C 8.43 0.16

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a) Calculate the number of theoretical plates for each compound and average number of

theoretical plates for the column. 

The number of theoretical plate is calculated using equation

For A: = .. 

= , 

For B: = .. 

= , 

For C: = .. 

= , 

 Average=46,300 plates

The average number of plates is 46,300 plates.

b) Calculate average height of a theoretical plate.

The height of a theoretical plate is equal to L/N where L is the length of the column and N is the

number of theoretical plates. Using the average number of theoretical plates gives theaverage height for a theoretical plate as

=   

=    = .  

(20 marks)