Qualification > Sciences
ALL CIE CHEMISTRY DOUBTS HERE !!
Amelia:
Example question - 20g of butanedioic acid (BDA) was shaken with a mixture of 100 cm3 ether and 100 cm3 water at 25oC. After titration with standard sodium hydroxide the concentration of the acid was found to be 0.024 mol dm-3 in ether and 0.16 mol dm-3 in water.
(a) Calculate the partition coefficient Kp for butanedioic acid between ether/water.
Kp = [BDA(ether)] / [BDA(water)] = 0.024 / 0.16 = 0.15
(b) If 10g of BDA had been shaken with 50 cm3 of each solvent at 25oC, what value would expect for Kp if the layers were again analysed?
0.15, since the partition law states the ratio of concentrations remains constant at constant temperature.
(c) If 10g of butanedioic acid was dissolved in 50 cm3 of ether at 25oC, calculate how much of the acid can be extracted with 50 cm3 of water.
If we call x the mass in g extracted into water, V the liquid volumes, and expressing the concentrations in gcm-3, substitution into the partition expression gives ...
Kp =
[(10-x)/Vether] (10-x)
---------------------- = ------- (since V's cancel out)
[x/Vwater] x
Rearranging: 0.15 x = 10 - x, so 1.15x = 10
therefore x = 10/1.15 = 8.7g of BDA extracted.
--- Check the past papers for more questions... or perhaps, the application booklet.
iluvme:
Please explain why Kc does not change with a change in concentration.
Master_Key:
--- Quote from: i<3SL on March 09, 2011, 11:42:48 am ---Please explain why Kc does not change with a change in concentration.
--- End quote ---
If you are changing the concentration of both the reactants then it will change.
If you are changing the concentration of one substance then it seems obvious that one of the reactant is in excess and does not react.
iluvme:
--- Quote from: Master_Key on March 09, 2011, 01:08:47 pm ---If you are changing the concentration of both the reactants then it will change.
If you are changing the concentration of one substance then it seems obvious that one of the reactant is in excess and does not react.
--- End quote ---
Thanks :)
Master_Key:
--- Quote from: i<3SL on March 12, 2011, 12:32:24 pm ---Thanks :)
--- End quote ---
You are welcome.
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