Question

Number of equivalents of $HCl$ present in $100mL$ of its solution whose $pH$ is $4$:

• A. ${10}^{-4}$
• B. ${10}^{-3}$
• C. ${10}^{-2}$
• D. ${10}^{-5}$

Answer 1

Answer: (D)

${10}^{-5}$

HCl is monobasic acid. Hence, its equivalent mass is equal to its molar mass and the number of equivalents are equal to the number of moles.
$pH=-log\left[H^{+}\right]=4$
$\left[H^{+}\right]={10}^{-4}M$
1 L of solution contains ${10}^{-4}$ moles of hydrogen ions.
100 mL of solution (0.1 L) will contain ${10}^{-4}\times \frac{100}{1000}={10}^{-5}$ moles of hydrogen ions.
This is also equal to the number of equivalents of HCl present in 100 mL of solution of pH 4.