Question

The pH of 0.1M monobasic acid with dissociation constant ${10^{ - 3}}$ will be:

Answer 1

Hint: A monobasic acid is also known as monoprotic acid which is able to donate one proton per molecule, and has only one hydrogen atom. First find the value of $p{K_a}$ using the dissociation constant. pH of an acid is the sum of $p{K_a}$ and the logarithm of ratio of concentrations of salt and acid. The concentration of acid is already given and the concentration of the mono-basic salt is 1. Substitute these values and find the pH

Step by step answer: We are given that molarity of a monobasic acid is 0.1M and its dissociation constant is ${10^{ - 3}}$
$p{K_a}$ is used to indicate the strength of an acid and the value of $p{K_a}$ is the negative logarithm of the acid dissociation constant.
$
  p{K_a} = - {\log _{10}}{K_a} \\
  {K_a} = {10^{ - 3}} \\
   \to p{K_a} = - {\log _{10}}\left( {{{10}^{ - 3}}} \right) \\
  \left( {\because \log {a^m} = m\log a;{{\log }_a}a = 1} \right) \\
   \to p{K_a} = - \left( { - 3} \right){\log _{10}}10 \\
   \to p{K_a} = 3 \times 1 \\
  \therefore p{K_a} = 3 \\
 $
Therefore the value of $p{K_a}$ is 3.
The pH, which is also known as the potential of hydrogen, is a scale used to specify acidity or basicity of an aqueous solution.
pH is defined as the sum of $p{K_a}$ and the logarithm of ratio of concentration of the salt and concentration of the acid.
$pH = p{K_a} + \log \dfrac{{\left[ {Salt} \right]}}{{\left[ {Acid} \right]}}$
The value of $p{K_a}$ is 3, the concentration of salt is 1M and concentration of acid given is 0.1M.
Substituting the values in the above formula, we get
$
  pH = p{K_a} + \log \dfrac{{\left[ {Salt} \right]}}{{\left[ {Acid} \right]}} \\
  p{K_a} = 3,\left[ {Salt} \right] = 1M,\left[ {Acid} \right] = 0.1M \\
   \to pH = 3 + {\log _{10}}\dfrac{1}{{0.1}} \\
   \to pH = 3 + {\log _{10}}10 \\
   \to pH = 3 + 1 \\
  \therefore pH = 4 \\
 $
The pH of the given monobasic acid with 0.1M is 4.


Note: If the value of pH is lower than $p{K_a}$ then the acid will be protonated (addition of hydrogen ions) and if the value of pH is greater than $p{K_a}$ then the acid will be deprotonated (losing of hydrogen ions).
Acidic solutions generally have lower pH values than basic solutions. pH below 7 is considered to be an acid and above is considered to be a base. And if the pH is 7 then the solution is neutral. Water is a neutral solution.