Question

Assertion :pH of acidic solution is always below 7 at ${25}^{o}C$. Reason: At ${25}^{o}C$, the pH of ${10}^{-8}MHCl$ is 8.

• A. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
• B. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion
• C. Assertion is correct but Reason is incorrect
• D. Both Assertion and Reason are incorrect

Answer 1

The correct option is C Assertion is correct but Reason is incorrect

The pH of acidic medium is always below 7 and that of bases is above 7 and neutral solutions have a pH of 7 at ${25}^{o}C$.

$pH=-lo{g}_{10}\left[H^{+}\right]$.

Thus pH is the negative log of the concentration of $H^{+}$ (hydrogen ions). Acids have a tendency to donate hydrogen ions and hence on their dissociation, we get hydrogen ions and a conjugate base. Thus, the total concentration of hydrogen ions is more than that of pure water concentration of hydrogen ions i.e (${10}^{-7}$).

Hence, when we take the negative log of the concentration of hydrogen ions for acids we get pH always lesser than 7 (which is for neutral compounds like water) as the concentration of hydrogen ions is more ( $\therefore$ the pH is less).

Here, the assertion is correct but the reason is incorrect.

Hence, the correct answer is option (C).