Question

Which of the following will produce the solution with the lowest pH?

(A) 0.2 M HCl(B)  0.5 M HF(C)  0.2 M KOH(D) 0.5 M HClO4(E)  There is not enough information to predicT

please help? im kind of lost

Answer 1

Which of the following will produce the solution with the lowest pH?

1. 0.2 M HCl(B) 0.5 M HF(C) 0.2 M KOH(D) 0.5 M HClO4(E)

Lowest pH would be of 0.5M HClO4 explained as follows,

1. 0.2M HCl, HCL is a strong acid and it dissociate completely to give following ion

HCl   àà               H⁺         +         Cl^-

0.2 moles/L---------0 --------------0------Initially

0.2 moles/L--------0.2---------------0.2-----Equilibrium

               pH = -log [H⁺]

               pH= -log 0.2 = 0.6990 = 0.7 approx

1. 0.5 M HF

HF             àà         H⁺         +         F^-

0.5----------------------0--------+ ------0-----Initial

Ka= 7.1 * 10^-4

Ka= [H⁺][Cl^-1]/[HF] = 7.1 * 10^-4

7.1 * 10^-4 = x²/ 0.5-x

               x= [H⁺] = 0.018

               pH = -log [H⁺]

               pH = -log 0.018 = 1.72

Hydrofluoric acid is classified as a weak acid because of its lower dissociation constant compared to the strong acids. It ionizes in aqueous solution in a similar fashion to other common acids.

HF + H₂O  H₃O⁺ + F⁻

HF is the only hydrohalic acid that is not considered a strong acid, i.e. it does not fully ionize in dilute aqueous solutions. When the concentration of HF approaches 100%, the acidity increases dramatically because of homoassociation:

3 HF H₂F⁺ + FHF⁻

The bifluoride (FHF−) anion is stabilized by the very strong hydrogen–fluorine hydrogen bond.

1. pH of 0.2M KOH

KOH àà   K⁺(aq)   +   OH^-(aq)

0.2mol L^-1 KOH produces 0.2mol L^-1 OH^- since KOH is a strong base that fully dissociates

Calculate the pOH:

pOH = -log10[OH-]

pOH = -log10[0.2] = 0.7

Calculate pH:

pH = 14 - pOH

pH = 14 - 0.7 = 13.3

1. 0.5M HCLO4

Perchloric acid, a superacid, is one of the strongest Brønsted-Lowry acids. Its pKa is −10. It provides strong acidity with minimal interference because perchlorate is weakly nucleophilic (explaining the high acidity of HClO4). It dissociate completely giving H⁺ and ClO₄^- ion.

HClO4           àà       H⁺   +      ClO₄^-

0.5 moles/L-------------0.5---+------0.5

             pH = -log [H⁺]

             pH = -log 0.5 = 0.3 approx

             pH= 0.3