Question

Which option follows that Formic acid is a weak acid and hydrochloric acid is a strong acid?
A. $[O{H^ - }]$ of $0.0{\text{ }}1MHCl(aq)$ will be less than that of $0.0{\text{ }}1MHCOOH(aq)$.
B. Solution containing $0.1MNaOH(aq)$ and \[0.1MHCOONa\left( {aq} \right)\] a buffer solution.
C. pH of ${10^{ - 9}}MHCl(aq)$ will be approximately $7$ at ${25^o }C$.
D. pH of a solution is formed by mixing equimolar quantities of $HCOOH$ and $HCl$ will be less than that of a similar solution formed from \[\left[ {HCOOH} \right]\]and \[\left[ {HCOOHNa} \right].\]

Answer 1

Hint: The product of hydrogen ions concentration and hydroxide ion's concentration is always constant.

 Complete step by step answer: As given in the question, formic acid is weak acid it means it have a lower concentration of hydrogen ion $[{H^ + }]$ on the other hand, Hydrochloric acid $[HCl]$ is a strong acid, which mean it have high concentration of hydrogen ions. As given above,
$[{H^ + }] \times [O{H^ - }] = CONSTANT$
Therefore, if formic acid is having low Hydrogen ion Concentration, it must have a higher concentration of $[O{H^ - }]$ is hydroxide Ion to counterbalance the product. Thus, $[O{H^ - }]$ of ${\text{ }}[0.0{\text{ }}1MHCl(aq)$ will be less than that of $0.0{\text{ }}1MHCOOH(aq)$. Also, we know that HCl is a strong acid and hence it will have an acidic pH that is less than $7$ . But, if we take ${10^{ - 9}}MHCl$ whose concentration is very low. The pH will be calculated on the basis of the number of $[{H^ + }]$ ions present in the solution. Due to lower concentration of $[{H^ + }]$ in ${10^{ - 9}}MHCl$,$[{H^ + }]$ ion from water will also be considered this will give us nearly $7 pH$ at ${25^o }C$ .
Additional information: For calculating pH of solution$(aq)$, we need to know the concentration of ${H_3}{O^ + }$ ions present in moles per liter that is Molarity expression for calculating pH:
$pH = - \log [{H_3}{O^ + }]$
Hence, option (A) $[O{H^ - }]$of $0.0{\text{ }}1MHCl(aq)$ will be less than that of $0.0{\text{ }}1MHCOOH(aq)$ and option (C) pH of ${10^{ - 9}}MHCl$ will be approximately $7$ at ${25^o}C$ both are correct.

Note:
The concentration of hydrogen and hydroxide ions always counterbalance each other if $[{H^ + }]$ions are more, $[O{H^ - }]$ Ions Will be less and vice versa.