The carboxylate group of acetic acid has a pKa of about 4.7. In an acetic acid s

Question

The carboxylate group of acetic acid has a pKa of about 4.7. In an acetic acid solution in water, approximately what percentage of the acetic acid molecules are protonated and neutral in charge at pH 5.7 ? Please show steps.

Explanation / Answer

ACIDITY CONSTANT As you know, acidic stuffs dissolve in polar solvents like water to dissociate itself although dissociation run partially. In effect, there are more than a chemical equilibrium related to every acidic chemical. For everyone, it may estimate the chemical constant of dissociation in aqueous media at defined temperature. Treating on general acidic stuff, HA be undissociated form while A- be its anion or dissociated form : chemical equilibrium binds the ions concentrations Ka = |H+| * |A-| / |HA| where it appears also |H+| : everytime you know |H+|, you may estimate pH = - log (|H+|). CHEMICALS The chemicals treated are Acetic Acid (CH3COOH) and Alanine Hydrochloride (Cl-,+H3NCH(CH3)-COOH). As you know, Acetic Acid is the volatile compound responsible for smelling and taste of vinegar and it results clearly an acidic stuff. On another place, Alanine Hydrochloride is the compound formed once Alanine (H2NCH(CH3)COOH) meet Hydrochloric Acid (HCl) : you have to think Alanine Hydrochloride like a salt having acidic nature since it bears a STRONG ACID (HCl) AGAINST A WEAK BASE (H2NCH2(CH3)COOH). Now I write the chemical equations CH3COOH --> CH3COO- + H+ so Acidity Constant result higher the more stable is its anion (Acetate ion). The lonely factor acting to get stability to acetate results orbitals ibridization across two Carbon/Oxygen chemical bonds. Treating about Alanine Hydrochloride +H3NCH2(CH3)COOH --> +H3NCH2(CH3)COO- + H+ so Acidity Constant result higher the more stable is its anion (Dipolar Ion defined ZWITTERION). Besides orbitals ibridization across two Carbon/Oxygen chemical bonds, now there is also Zwitterionic Nature which gets electroneutrality to intermediate despite negative chagre hosted on Acetate ion. CONCLUSION Alanine Hydrochloride results a stronger acid than Acetic Acid on the basis of stability and electrical charge of its dissociated form. I hope this helps you.

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