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If a buffer solution is 0.120 M in a weak acid (Ka = 3.5 × 10-5)...

If a buffer solution is 0.120 M in a weak acid (Ka = 3.5 × 10-5) and 0.500 M in its conjugate base, what is the pH?
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Answer #1
Concepts and reason

Concentration of a weak acid is related to its pк,
and pH of a solution as shown in the following Henderson -Haselbach equation.

[salt]
pH pklogSalt]
[acid]

Here, К,
a
is the acid dissociation constant; salt]
, and[acid]
are the concentration terms of the salt and the acid respectively. This equation is applicable to a system that contains weak acids or weak bases only.

Because the acid is a weak acid, we can use the above equation to calculate the pH.

The value of pH for the solution can calculate by substituting the concentration values in the formula.

Fundamentals

An aqueous solution that contains a mixture of a weak acid and its conjugate base or vice versa is called as buffer solution.

Conjugate base differs with a proton(н)
to its acid.

Consider the weak acid as HA. It is dissociated as shown below.

Н +A
НА

The Henderson -Haselbach equation can be written as follows:

[A]
pH pKalog
[HA

Calculate the value of К,
a
and then pH of the solution by substituting the concentrations in the equation.

pK log K
-log (3.5x10
= 4.45

Now calculate the pH as follows:

[A]
pH pK,logHA
[HA
=4.45+log0.500
0.120
4.45 0.6197
= 5.06

Ans:

Thus, the pH of the solution is 5.06
.

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