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Question: Which option follows that Formic acid is a weak acid and hydrochloric acid is a strong acid? A. \(...

Which option follows that Formic acid is a weak acid and hydrochloric acid is a strong acid?
A. [OH−][O{H^ - }] of 0.0 1MHCl(aq)0.0{\text{ }}1MHCl(aq) will be less than that of 0.0 1MHCOOH(aq)0.0{\text{ }}1MHCOOH(aq).
B. Solution containing 0.1MNaOH(aq)0.1MNaOH(aq) and 0.1MHCOONa(aq)0.1MHCOONa\left( {aq} \right) a buffer solution.
C. pH of 10−9MHCl(aq){10^{ - 9}}MHCl(aq) will be approximately 77 at 25oC{25^o }C.
D. pH of a solution is formed by mixing equimolar quantities of HCOOHHCOOH and HClHCl will be less than that of a similar solution formed from [HCOOH]\left[ {HCOOH} \right]and [HCOOHNa].\left[ {HCOOHNa} \right].

Explanation

Solution

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+][{H^ + }] on the other hand, Hydrochloric acid [HCl][HCl] is a strong acid, which mean it have high concentration of hydrogen ions. As given above,
[H+]×[OH−]=CONSTANT[{H^ + }] \times [O{H^ - }] = CONSTANT
Therefore, if formic acid is having low Hydrogen ion Concentration, it must have a higher concentration of [OH−][O{H^ - }] is hydroxide Ion to counterbalance the product. Thus, [OH−][O{H^ - }] of  [0.0 1MHCl(aq){\text{ }}[0.0{\text{ }}1MHCl(aq) will be less than that of 0.0 1MHCOOH(aq)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 77 . But, if we take 10−9MHCl{10^{ - 9}}MHCl whose concentration is very low. The pH will be calculated on the basis of the number of [H+][{H^ + }] ions present in the solution. Due to lower concentration of [H+][{H^ + }] in 10−9MHCl{10^{ - 9}}MHCl,[H+][{H^ + }] ion from water will also be considered this will give us nearly 7pH7 pH at 25oC{25^o }C .
Additional information: For calculating pH of solution(aq)(aq), we need to know the concentration of H3O+{H_3}{O^ + } ions present in moles per liter that is Molarity expression for calculating pH:
pH=−log⁡[H3O+]pH = - \log [{H_3}{O^ + }]
Hence, option (A) [OH−][O{H^ - }]of 0.0 1MHCl(aq)0.0{\text{ }}1MHCl(aq) will be less than that of 0.0 1MHCOOH(aq)0.0{\text{ }}1MHCOOH(aq) and option (C) pH of 10−9MHCl{10^{ - 9}}MHCl will be approximately 77 at 25oC{25^o}C both are correct.

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