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A buffer solution made with NH3 and NH4Cl has a pH of 10.0. Which procedure(s) could...

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lak-86 | Student, Undergraduate | Salutatorian

Posted August 15, 2013 at 9:54 AM via web

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A buffer solution made with NH3 and NH4Cl has a pH of 10.0.

Which procedure(s) could be used to lower the pH?
1. adding HCl
2. adding NH3
3. adding NH4Cl

(A) 1 only

(B) 2 only
(C) 1 and 3 only

(D) 2 and 3 only

1 Answer | Add Yours

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llltkl | College Teacher | Valedictorian

Posted August 15, 2013 at 11:59 AM (Answer #1)

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`pH=K_w/(pOH)`

Thus, lowering the pH means increasing pOH.

The Henderson-Hasselbalch equation for pH of a basic buffer solution is:

pOH=pK_b+log[salt]/[base]

It therefore follows that, pOH can be increased by

i) increasing the concentration of the salt

ii) lowering the concentration of the base.

Addition of a small amount of HCl has no effect on the pH of the solution as the buffer has the apacity to contain its effect.

However, if the amount of HCl added exceeds certain limit, called the buffer capacity of the solution, pH is lowered.

Addition of salt (NH4Cl) have the same effect on the pH.

In summary, if we are to lower the pH of the buffer solution, option C) should be the choice.

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