Question

Useful buffer range of weak acid HA \[{K_a} = {10^{ - 5}}\] is:
A. 5 to 7
B.4 to 6
C.3 to 5
D.None of these

Answer 1

vHint:\[{K_a}\]is the acid dissociation constant. It helps to determine the strength of the acid. The \[pH\]range where the buffer neutralizes the added acids and bases is called buffer range. While doing so, the buffer tries to maintain the constant\[pH\].
Formula used:
\[
  pH = p{K_a} + \log \dfrac{{[conjugateBase]}}{{[acid]}} \\
  p{K_a} = - {\log _{10}}{K_a} \\
 \]

Complete step by step answer:
 Given,\[{K_a} = {10^{ - 5}}\]
Using the relation, \[p{K_a} = - {\log _{10}}{K_a}\]
We get \[p{K_a} = - {\log _{10}}{K_a} = - {\log _{10}}{10^{ - 5}} = 5\]
Now, we will use this value to determine the potential of hydrogen\[pH\]by using the formula:
\[pH = p{K_a} + \log \dfrac{{[conjugateBase]}}{{[acid]}}\]
For an acid HA,
Case 1: \[\dfrac{{[conjugateBase]}}{{[acid]}} = \dfrac{1}{{10}}\]
Substituting this value,
\[pH = p{K_a} + \log \dfrac{1}{{10}}\]
We get,\[pH = 5 + \log 0.1\]
\[pH\]=5-1=4 …(I)
Case 2: \[\dfrac{{[conjugateBase]}}{{[acid]}} = \dfrac{{10}}{1}\]
Substituting this value
\[pH = 5 + \log \dfrac{{10}}{1}\]
We get,
\[pH\]=5+1=6 …..(II)
So, Useful buffer range of weak acid is 4 to 6.

Hence, option B. (4 to 6) is the correct option to the given question.

Note:
Lesser the value of\[p{K_a}\], stronger is the acid. Temperature and concentration can affect the\[p{K_a}\] Buffer usually maintains the\[pH\] of the system and resists the change in the concentration of \[{H^ + }\] ions in the system or solution.