Deck 24: Introduction to Analytical Separations

Question 1

Which is NOT true when scaling up preparative chromatography?

A)Keep column length constant.
B)Cross-sectional area of a column is inversely proportional to the mass of analyte.
C)Maintain constant linear velocity.
D)Sample volume applied to column is proportional to mass of analyte.
E)If you change column length,mass of sample can be increased in proportion to total length.

Answer 1

Cross-sectional area of a column is inversely proportional to the mass of analyte.

Question 2

Calculate the weak acid,HA,concentration that remains in the aqueous phase when 50.0 mL of 0.184 M HA at pH = 4.75 is extracted with 50 mL of toluene three times.The partition coefficient for the extraction is 5.17,favoring toluene.The K_a for HA is 8.0 x 10⁻⁴.

Answer 2

72.5% remains or 0.133 M HA remains in the aqueous phase.

Question 3

A solute is to be extracted using 150 mL of solvent.Of the options below,which will extract the maximum amount of solute?

A)extract with 30 mL five times
B)extract with 37.5 mL four times
C)extract with 75 mL twice
D)extract once with the entire volume
E)extract with 50 mL three times

Answer 3

extract with 30 mL five times

Question 4

For two separated solutes the  = 1.10 and the k₂ = 4.38.How many theoretical plates are required to achieve a resolution of 1.5?

Question 5

Which statements are NOT true regarding chromatography?
I The stationary phase is typically a liquid bonded to the inside surface of a capillary or the surface of solid packed in the column.
II The mobile phase entering the column is the eluate.
III Mobile phase is the solvent (gas or liquid)that moves through the column.
IV Eluent is the process of passing mobile phase though a column.
V Columns are either open tubular or packed.

A)I and V
B)II and IV
C)I,II,and III
D)III and V
E)I,IV,and V

Question 6

Which is NOT true for solute retention?

A)Retention time,t_r,is the elapsed time between injecting the sample to the arrival of a sample component at the detector minus the dwell time between the end of the column and the detector.
B)Adjusted retention time is t_r^' = t_r - t_m,where t_r is the retention time and t_m is the time for mobile phase or unretained solutes to pass through the column.
C)The relative retention, =

,is the ratio for the adjusted retention time for two components of the sample,where t_r2^' > t_r1^' so that  > 1.
D)The retention factor,k =

,is the time,t_r,required to elute a component minus the time,t_m,for mobile phase to pass through the column divided by t_m.
E)The relative retention,,also equals

,where K₂ and K₁ are the partition coefficients for components 2 and 1.

Question 7

The partition coefficient is modified when the pH of the aqueous phase is adjusted or when a chelating agent is added.The partition coefficient is renamed the distribution coefficient.What is the impact on the extraction of a weak acid into an organic solvent as the pH of the aqueous phase increases?

A)As the pH increases the value of D decreases and the percentage weak acid left in the aqueous phase decreases.
B)As the pH increases the value of D increases and the percentage weak acid left in the aqueous phase increases.
C)As the pH increases,the value of D decreases and the percentage weak acid left in the aqueous phase decreases.
D)As the pH increases the value of D increases and the percentage weak acid left in the aqueous phase decreases.
E)As the pH increases the value of D decreases and the percentage weak acid left in the aqueous phase increases.

Question 8

The longer an analyte remains on the column,the broader the peak becomes.Peaks widen with time on column due to :

A)effusion.
B)diffusion.
C)adsorption.
D)absorption.
E)viscosity.

Question 9

Calculate the resolution for two chromatographic peaks that elute at t_r1 = 1770 s and t_r2 = 1800 s with base peak widths of w₁ = 51 s and w₂ = 57 s.

Question 10

The resolution between two peaks is defined as resolution = .Which variable is INCORRECTLY defined?

A)t_r is the retention time.
B)V_r is the retention volume.
C)w_av is the average peak width measured at the base of each peak.
D)w_1/2av is the average peak width measured at the base of each peak,divided by 2.
E)resolution is the separation between two adjacent peaks.

Question 11

Which form of chromatography is INCORRECTLY defined?

A)adsorption chromatography - solute is adsorbed into solid particles
B)partition chromatography - solute equilibrates between the stationary liquid and the mobile phase
C)ion-exchange chromatography - solute ions of the opposite charge are attracted to the stationary phase
D)molecular exclusion chromatography - solute is separated based on size
E)affinity chromatography - solute binds to a molecule specific for the solute

Question 12

____________________ is a plot of detector response versus retention time.

A)Spectrum
B)Electrophoretogram
C)Chromatogram
D)Chromospectrum
E)Elution response

Question 13

Calculate the number of theoretical plates and plate height for a peak with a retention time of 1295 seconds,a width at the baseline of 27 seconds and elutes from a 15 m column.

Question 14

Which comparison of open tubular columns to packed columns is INCORRECT?

A)Open tubular columns have higher resolutions than packed columns.
B)Open tubular columns have increased linear velocity or a longer column or both.
C)Packed columns have decreased sensitivity.
D)Packed columns have longer analysis times.
E)Packed columns have a lower sample capacity.

Question 15

The van Deemter equation describes plate height in terms of constants A,B,C,and the linear velocity,u_x.Which statement is NOT true for the van Deemter equation?

A)A takes into account multiple pathways through the column.The value of A is column specific and independent of linear flow.
B)B takes into account longitudinal diffusion of the analyte in the mobile phase.The value of B is column specific and the impact of B on the plate height is inversely proportional to the linear velocity.
C)C takes into account equilibrium time between the stationary and mobile phase.The value of C is column specific and the impact of C on plate height is proportional to the linear velocity.
D)To minimize plate height,the optimal flow rate is the minima for the plot of

Versus flow rate.
E)Linear velocity is the flow rate of the mobile phase.

Question 16

The relative retention between two solutes,1 and 2,is  = 1.35.If the adjusted retention time for solute 2 is 293 s,what is adjusted retention time for solute 1?

A)396 s
B)217 s
C)189 s
D)241 s
E)344 s

Question 17

____________________ is the physical transfer of solute from one phase to another.

A)Partitioning
B)Extraction
C)Activated transport
D)Sieving
E)Miscibility

Question 18

Suppose that the partition coefficient for an amine,B,is K = 3.0 and the acid dissociation constant for BH⁺ is K_a = 1.0 x 10⁻⁹.If 50.0 mL of 0.010 M aqueous amine is extracted with 100.0 mL of solvent two times,what percentage amine remains in aqueous solution if the pH of the aqueous phase is 9.00?

Question 19

The observed variance, ,for a peak is the sum of the variances from all contributing factors.Which of the following is not a contributing factor to ?

A)
B)
C)
D)
E)

Question 20

Which is true for plate height?
I The smaller the plate height,the narrower the peaks.
II The larger the plate height,the better separation between peaks.
III Plate height can be calculated from the column length,L,and the number of theoretical plates,N.
IV For fixed number of theoretical plates,N,the longer a solute is on column,the wider the peak will become.

A)I,II,and IV
B)II and IV
C)I,III,and IV
D)II only
E)IV only