Question

On adding which of the following, the pH of $20$ ml of $0.1N$ $HCl$ will not alter?

• A. $1$ ml of $1N$ $HCl$
• B. $20$ ml of distilled water
• C. $1$ ml of $0.1N$ $NaOH$
• D. $500$ ml of $HCl$ of pH = $1$

Answer 1

Answer: (D)

$500$ ml of $HCl$ of pH = $1$

On adding $500$ ml of $HCl$ of pH =$1$, the pH of $20$ ml of $0.1NHCl$ will not alter.
When pH is $1$, $\left[H^{+}\right]={10}^{-pH}={10}^{-1}$ M. Since $HCl$ is monobasic acid, its molarity is equal to its normality. Hence, $500$ ml of $HCl$ of pH =$1$ corresponds to $0.1N$ solution. So when $500$ ml of $0.1NHCl$ are added to $20$ ml of $0.1NHCl$, the pH of $20$ ml of $0.1NHCl$ will not alter.