Final tmw! HELP PLEASE!!! A buffer is required that has a pH of 4.

Question

1) A buffer is required that has a pH of 4. Which of the following acid/base pairs would be the most suitable? You may need the dissociation constants:

HCOOH Ka=1.8E-4

CH3COOH Ka=1.8E-5

H3AsO4 Ka1=6.0E-3, Ka2=1.1E-7, Ka3=3E-12

C5H5N Ka=1.7E-9

NH3 Ka=1.8E-5

A) H3AsO4/NaH2AsO4

B) HCOOH/NaHCOO

C) C5H5N/C5H5NHCl

D) CH3COOH/KCH3COO

E) NH4Cl/NH3

answer: B

2) Which of the following mixtures would NOT be described as a buffer solution?

A) 0.15 M NH4Cl and 0.25 M NH3

B) 0.20 M NaOH and 0.40 M CH3COOH

C) 0.25 M HNO2 and 0.15 M KNO2

D) 0.25 M HNO3 and 0.15 M KNO3

E) 0.30 M NH3 and 0.15 M HCl

Answer: D

How do you do this and others like it?

THANKS!!!

Answer 1

1) A buffer is required that has a pH of 4. Which of the following acid/base pairs would be the most suitable? You may need the dissociation constants:

HCOOH Ka=1.8E-4  pKa = 3.74

CH3COOH Ka=1.8E-5 pKa = 4.74

B) HCOOH/NaHCOO

answer: B is right, an Ideal buffer has a pKa  equal to the desired pH,

& formic acid came the closest

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2) Which of the following mixtures would NOT be described as a buffer solution?

D) 0.25 M HNO3 and 0.15 M KNO3

Answer: D, yes because it is not a weak acid &  its conjugate base, HNO3 is a strong acid