In steps 7 and 14: in order to keep Al(OH)3 as a solid, it is necessary to keep

Question

In steps 7 and 14: in order to keep Al(OH)3 as a solid, it is necessary to keep the pH relatively close to 7. The approach is to buffer the pH at approximately pH=9 using NH3/Nh4+ buffer.
What is the calculated pH of this bigger if it has way concentrations of NH3 and NH4+?
Where do the weak acid and conjugate base come from and what did you add to the solution (2 chemicals) to make the buffer in these steps?
step 7:

Step 14:

7. a) To the solution from step 3, add 6 M NH, until the solution is basic to indicator paper, then add 5 drops more. Stir the solution well. Centrifuge and decant the solution. own: broun color ushun tur unknown: white ppt formed b) Wash the solid from step 7a well with 10 drops of pH 9 buffer solution. Mix well then centrifuge Discard the wash and the solution.

Explanation / Answer

As in those steps of your experiment ,you are dissolving the precipitate in 6M HCl and adding 6M NH3 until basic , so pH can be calculated using henderson-hasselbach eqn ,

pH=pka + log [base]/[acid]

Here NH3+HCl -->NH4+ Cl- (neutralization rxn) [weak acid generation]

pka of NH3/NH4+ system=9.3

Consider complete neutrazation of HCl , [HCl]=[NH4+]

to maintain pH=9.0

pH=pka+log [NH4+]/[NH3] gives

9.0=9.3+log [NH4+]/[NH3]

10^-0.3=0.5=[NH4+]/[NH3]

or, [NH3]=2[NH4+] =2[HCl]

the amount of NH3 must be half the amount of HCl so as t reach the half -equivalence point to get a pH=9.0 approximately

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