Please be clear with your answer.... A mixture of two compounds, one with pKa=3
ID: 794680 • Letter: P
Question
Please be clear with your answer....
A mixture of two compounds, one with pKa=3 and the other with pKa=10 are dissolved in 150.0 ml of ether and placed in a separatory funnel. This solution is extracted two times with 50.0ml portions of the 5% bicarb solution just prepared. This uses up all of the bicarb solution. Which of the two acids end up in the aqueous layer?
The salt extracted into the aqueous layer above is precipitated by neutralizing the bicarbonate. How many ml of the 4.0 M HCl is needed to do this?
How would you go about extracting the other acid that remains in the ether layer?
Explanation / Answer
You have a mixture of an strong acid (pKa=3) and a weak acid (pKa=10)
If you add bicarb solution, the stronger will react first (the one with pKa=3) to form the salt. the salt is completely soluble in water, then, in the aqueous layer you have the salt of the acid with pKa=3.
In the organic layer will remmaing the weak acid.
2 portions of 50 mL of 5% NaHCO3= 100mL x(5g/100mL)= 5 g NaHCO3
5 g NaHCO3 x (1 mol NaHCO3/84.07g )= 0.059 mol NaHCO3
HCl(aq) + NaHCO3(s) = NaCl(aq) + H2O(l) + CO2(g)
the you need 0.059 mol of HCl (note that you have 4.0 M HCl)
0.059 mol of HCl x (1 L/4.0 mol)= 0.01475 L = 14.75 mL
How would you go about extracting the other acid that remains in the ether layer?
If you want to extract the weak acid, you need a stronger base, for example NaOH.
if you add NaOH solution, you will form the salt of the weak acid, this will be soluble in water.
finally, if you add a HCl, until you neitralize all the NaOH, you will precipitate the weak acid.