Question

Calculate the $pH$ of a solution which is $0.1M$ in $HA$ and $0.5M$ in $NaA$. ${{K}_{a}}$ for $HA$ is $1.8\times {{10}^{-6}}$ .
A.3
B.4
C.6.4
D.10

Answer 1

You should know how to calculate the $pH$ using Henderson equation. You should know a brief about $pH$ . It is defined as a scale used to measure acidity and basicity. ${{K}_{a}}$ , that is the dissociation constant of acid.
Formula Used:
$pH=p{{K}_{a}}+\log \dfrac{[salt]}{[acid]}$
where, $pH$ is the acidity of a solution and ${{K}_{a}}$ is dissociation constant of acid.

Complete step by step answer:
Here, it is given that the concentration of $HA$ is $0.1M$ and concentration of $NaA$ is $0.5M$.
Here, the dissociation constant of acid $({{K}_{a}})=1.8\times {{10}^{-6}}$
According to the formula, $pH=p{{K}_{a}}+\log \dfrac{[salt]}{[acid]}$
Substituting the values given, we get,
$pH=(1.8\times {{10}^{-6}})+\log \dfrac{[0.5]}{[0.1]} \\\ pH=5.74+0.69 \\\ \Rightarrow pH=6.44 \\\$
Hence, the correct option is C. $6.4$.

Additional information:
$pH$ is defined as a scale which is used to measure acidity and basicity of an aqueous solution or liquid. The $pH$ less than seven considers an acidic solution, whereas $pH$ more than seven considers a basic solution. And if the $pH$ is equal to seven, the solution is considered neutral.
$pH=-\log [{{H}^{+}}]$
where, ${{H}^{+}}$is the concentration of ${{H}^{+}}$ ions in the solution.
${{K}_{a}}$ is defined as dissociation constant of an acid. If the value of dissociation constant of an acid is high then, it is a strong acid, whereas if the value of dissociation constant of an acid is low then it is a weak acid.
$p{{K}_{a}}$ is defined as the negative base$10$ logarithm of the acid dissociation constant. If the $p{{K}_{a}}$ value is lower then, it is a stronger acid, compared to the acid with a value of $p{{K}_{a}}$ which is higher.
The formula used is Henderson equation.
$pH=p{{K}_{a}}+\log \dfrac{[salt]}{[acid]}$
Henderson equation is used to know the value of $pH$ of a buffer solution. Buffer is defined as which can resist a change in its $pH$.

Note: A simple buffer solution consists of salt of a conjugate base and an acid.
Buffers are of two types:
-Simple buffer: It is defined as the salt of a weak acid and weak base. For example, $C{{H}_{3}}COON{{H}_{4}}$
-Mixed buffer: It can be acidic, and it can be basic. For example,
Acidic: $C{{H}_{3}}COOH+C{{H}_{3}}COONa$
Basic: $N{{H}_{3}}+N{{H}_{4}}Cl$
-The equilibrium between the weak acid and conjugate base, it allows the solution to resist change to pH when a strong base and acid is added. The buffer pH can be calculated using the Henderson equation.