What is the difference (in pH units) between the initial pH of a 0.20 M NH3 / 0.

Question

What is the difference (in pH units) between the initial pH of a 0.20 M NH3 / 0.20 M NH4Cl buffer to the pH of the buffer after the addition of 10.0 mL of 0.10 M HCl to 65.0 mL of the buffer?

A. there is no measurable difference between final and initial pH - that is why it is called a buffer
B. final pH is 0.16 pH units higher than original pH
C. final pH is 0.16 pH units lower than original pH
D. final pH is 0.07 pH units lower than original pH
E. final pH is 0.07 pH units higher than original pH

Explanation / Answer

What is the difference (in pH units) between the initial pH of a 0.20 M NH3 / 0.20 M NH4Cl buffer to the pH of the buffer after the addition of 10.0 mL of 0.10 M HCl to 65.0 mL of the buffer? (Ka of NH4+ at 25)

initially:

mmol of NH3 = MV = 0.20*65 = 13 mmol of NH3

mmol of NH4Cl = MV = 0.2*65 = 13 mmol of NH4Cl

Now,

after adding

mmol of HCl = MV = 10*0.1 = 1 mmol of H+

H+ reacts with NH3 to form NH4+

mmol of NH3 left = 13-1 = 12

mmol o fNH4+ formed = 13+1 = 14

this is a buffer, so substitute in buffer equation

pH = pKa + log(NH3/NH4+)

pKa = 14-pKb = 14--log(1.8*10^-5) = 9.25

pH = 9.25 + log(12/14)

pH = 9.1830

initial pH was:

pH = pKa + log(NH3/NH4+) = 9.25

change in pH:

dpH = 9.1830-9.25 = -0.067

due to the addition of HCl

pH must decrease, the change is 0.067 (decreasing)

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