Question

Which of the following statement(s) is/are true?

• A. The pH of ${10}^{-8}MHCl$ is 8
• B. The conjugate base of $H_{2}P{O}_4^{-}$ is $HP{O}_4^{2-}$
• C. If in an aqueous solution $[H^{+}{]}_{strongacid}<{10}^{-6}M$
  $\left[H^{+}\right]$ of water cannot be neglected.
  
• D. If in an aqueous solution $[O{H}^{-}{]}_{strongbase}<{10}^{-6}$ then in $\left[O{H}^{-}\right]$ of water can be neglected.
  

Answer 1

Answer: (B), (C)

The correct options are
B The conjugate base of $H_{2}P{O}_4^{-}$ is $HP{O}_4^{2-}$
C If in an aqueous solution $[H^{+}{]}_{strongacid}<{10}^{-6}M$

$\left[H^{+}\right]$ of water cannot be neglected.

a) Since $HCl$ is an acid so, its pH should be < 7.
Here, $[H^{+}{]}_{total}={10}^{-8}+{10}^{-7}$
(by acid) (by water)
So, $pH=-\log [H^{+}{]}_{total}=-log(1.1\times {10}^{-7})=6.95$
Hence, the pH of ${10}^{-8}MHCl$ is not 8 but 6.95.

b) Conjugate base is formed by the loss of proton from an acid. So conjugate base of $H_{2}P{O}_4^{-}$ is $HP{O}_4^{2-}$.
Since $HP{O}_4^{2-}$ is formed by the loss of proton from $H_{2}P{O}_4^{-}$. So it is correct.

c) If in an aqueous solution $[H^{+}{]}_{strongacid}<{10}^{-6}$ then the $\left[H^{+}\right]$ of water cannot be neglected. This is because, the concentration of $H^{+}$ of water becomes comparable to that of the concentration of $H^{+}$ from strong acid. So, both $H^{+}$ concentrations need to be considered.

d) If in an aqueous solution $[O{H}^{-}{]}_{strongbase}<{10}^{-6}$ then the $\left[O{H}^{-}\right]$ of water cannot be neglected. This is because, the concentration of $O{H}^{-}$ of water becomes comparable to that of the concentration of $O{H}^{-}$ from strong base. So, both $O{H}^{-}$ concentrations need to be considered.