Question

The gastric juice in our stomach contains enough HCl to make the hydrogen ion concentration about \[0.01mol{L^{ - 1}}\]. The pH of gastric juice is……
(A) 0.01
(B) 1
(C) 2
(D) 14

Answer 1

Hint: Hydrochloric acid is a monoprotic acid and it has only one acidic proton. We can find pH of any solution if we know the concentration of hydrogen ions present in the solution by following the formula.
\[pH = - \log \left[ {{H^ + }} \right]\]

Complete step by step solution:
We are given the information that gastric juice contains HCl. So, we know that it is a mineral acid and in aqueous media, it will dissociate into ions and one molecule of HCl will give one proton. The proton is nothing but the positively charged hydrogen ion.
Here, we are given the concentration of the Hydrogen ions present in the stomach. We know that pH of a solution can be found out easily by concentration of hydrogen ions in the solution. Now, using this concentration, we can easily find the pH of this solution by using the formula of pH shown as under.
Formula used: \[pH = - \log \left[ {{H^ + }} \right]\]………………..(1)
We know that
\[\left[ {{H^ + }} \right] = 0.01M\] as \[M = \dfrac{{mol}}{L}\]
Putting this value in equation (1),
\[pH = - \log \left[ {0.01} \right]\]
Now, find the log value and put it into the equation. The log value of (0.01) is (-2)
\[pH = - \left( { - 2} \right)\]
\[pH = 2\]
So, the correct answer is “Option C”.

Additional Information:
- In the formula \[pH = - \log \left[ {{H^ + }} \right]\], concentration of Hydrogen ion is always expressed in Molarity.
- The concept and formula of pH was given by Sorenson. We can find pH of any solution by the concentration of Hydrogen ions present in the solution and Concentration of Hydrogen ions can be found by pH of the solution.
- Lets see why \[M = \dfrac{{mol}}{L}\].
As we know that
 \[M = \dfrac{{{\text{Weight of solute(gm)}}}}{{{\text{Molecular weight of solute }} \times {\text{ Volume of solution(L)}}}}\]
but
\[\dfrac{{{\text{Weight of solute(gm)}}}}{{{\text{Molecular weight of solute}}}} = Mole\]
so, we can write that
\[M = \dfrac{{{\text{Moles of solute}}}}{{{\text{Volume of solution}}}}\]

Note: Do not forget the minus sign in the formula of pH as forgetting it may lead to errors. One should know how to interconvert units of concentration as this may create problems when concentration is not expressed in the required unit.