Determine the distance of the solvent front if the R_f value of a sample is 0.50
ID: 1007581 • Letter: D
Question
Determine the distance of the solvent front if the R_f value of a sample is 0.50 and in which the sample moved was 5.0 cm from the spotting point? Show all calculations Determine the distance in which the sample moved from the spotting point if the R_f value sample is 0.25 and the distance of the solvent front from the reference point (spotting polar cm? show all calculations if both experiments required ten minutes for completion, how do you compare the polarity of each sample in problem 7 and 8? "Briefly explain" if the same sample was used in both experiments in examples 7 and 8 above and in experiment 7, it required 10 minutes for the sample to move 5.0 cm and in experiment 8, it required 20 minutes for the same sample to move, 5.0 cm. what can be said about the polarity of the eluting solvents? "Briefly explain" when carrying out an analysis of a mixture, what do you expect to see when the TCL plate has been allowed to remain in the development chamber too long, so that the solvent front has exceeded the top of the TLC plate, "Briefly explain" why must the spot applied to a TLC plate be above the level of the developing solvent(eluant). "Briefly explain" after eluting halfway, a TLC plate showed two spots of R_f 0.25 and 0.26(AR_f = 0.1). the TLV plate was removed from the developing chamber, dried carefully, and returned to the developing chamber, what would you expect to see after the second development was completed in term of AR_f? "Briefly explain"Explanation / Answer
7.
Rf value = distance traveled by compound ÷ distance traveled by solvent front
Solvent front = distance traveled by compound/Rf value = 5/0.5 = 10
8.
Rf = 0.25, if solvent fron is 10cm
distance traveled by compound = Rf value * distance traveled by solvent front = 0.25*10= 2.5cm
9. both experiment requires 10min for completion,
first exp Rf= 0.5; second exp Rf= 0.25
Rf values are mainly dependent upon the nature of the adsorbent and solvent system; in order to determine whether an unknown compound is identical to a compound of known structure, it is necessary to run the two samples side by side on the same TLC plate having same concentration. Its a rule that low polarity compounds have higher Rf values and higher polarity compounds have lower Rf values.
From this knowledge we can say that compound in problem 7 is with Rf 0.5 is less polar than compound in question 8 which have Rf= 0.25
10.
Same compound were used in both experiment and in one exp. it takes 10min to travel 5cm and in another exp. it takes 20min to travel 5cm.
Since compound is same in both experiments but second one is taking 20 min to travel same distance , solvent in second experiment is very liss polar than in first experiment. If the polarity of solvent increases spot will run faster on TLC and reached 5 cm in less time duration.
11.
If we are analysing mixture of compounds with different polarity and hence different Rf values and we kept TLC plate in developing chember for too long so that the solven fron has exceeded the top of TLC plate, all sopts corresping to different solvents will apprear at the top of TLC plate in on line. Such results are of no use.
12.
TLC spot is always applied to the line above the level of developing solven because if spots are dipped in solvens or they touches the developing solvents they will diffused (pulled down) in the solvent and we wont get expected TLC results. Instaed if we keep them bit above developing solvent they will go up aling with rising developing solvent (capillary action) and they will differentiate properly depending upon their polarity.
13.
If we are separating mixture of two compound whoes Rf is known for perticular stationayr phase and solvent mixture then even after drying deltaRf value doesnt change it will stay same.
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