Help on Chem Problem--Buffers!?

Question

I don't understand the question : Which of the following mixtures gives a buffer with a pH less than 7.0? For CH3CooH, ka= 1.8 x 10^-5, and for NH3, kB= 1.8x10^-5.
The answer choices are:
a) 10 mL of .1 M NH3 + 10 mL of .1M HCL
b) 10 mL of .1 M Ch3CooH + 5.0 mL of .1 M NaOH
c) 10 mL of .1 M CH3COOH + 10 mL of .1 M NaOh
d) 10 mL of .1 M CH3Cooh + 10 mL of .1 M NH3
e) 10 mL of .1 M NH3 + 5mL of .1 M HCL


the answer is A, but i don't understand why..can someone help or explain it to me?thanks!

Answer 1

From theory and without resort to calculations -

a) This will yield 20 mls of a 0.05 M solution of NH4Cl. This is the salt of a strong acid and a weak base. It will be acidic and hence have a pH < 7.0

b) Acetic acid is in the excess and hence the solution will be acidic with a pH < 7.0

c) This will yield 20 mls of a 0.05 M solution of sodium acetate. This is the salt of a strong base and a weak acid. It will be basic and hence have a pH > 7.0

d) This will yield 20 mls of a 0.05 M solution of ammonium acetate. This is the salt of a weak acid and base of the same acidity and basicity. The salt will be neutral ie pH =7.0

e) The ammonia is in the excess and the solution will be alkaline ie pH > 7.0

The answer is (a) and (b).

Answer 2

i'd agree. The pKa of acetic acid is 4.seventy six therfore pH = pKa + log[base]/[Acid] pH = 4.seventy six + log(a million) pH = 4.;seventy six + 0 pH = 4.seventy six additionally explanation why they call them buffers is that the pH won't replace with the addition of an quantity of acid or base. It in fact certainly relies upon on the buffering means The buffering means is the flexibility of a buffer to stand up to adjustments in pH.. the optimal buffering means is obtained whilst the pH =pKa. The closer the buffered pH is to the pKa of the acid the greater suitable the buffering means. Therfore you will would desire to upload a solid quantity of acid or base to even touch the pH.

Answer 3

The answer is B, whatever your book says!
A produces just pure ammonium chloride solution, which, although it is acidic, is not a buffer.
In B the ethanoic acid is half neutralised, and the buffer will give a pH of 4.76.