Question

My homework question is Lactic Acid (Ch3CHOHCOOH) has a pKa of 3.86. Write the ionization equation fro lactic acid. Indicate the weak acid and the conjugate base for this buffer system. Calculate the ratio of conjugate base to weak acid at pH 3.86.

Answer 1

CH3CHOHCOOH(aq) --> molecular acid

CH3CHOHCOO-(aq) --> conjugate base, i.e. acid has donated its proton

H3O+(aq) --> proton in solution (plus water H2O molecule)

CH3CHOHCOOH(aq) + H2O(l) <-> CH3CHOHCOO-(aq) + H3O+(aQ)

Bronsted Lowry acid: any species that will donate H+ (protons) in solution, and makes pH lower (i.e HCl)

Bronsted Lowry base: any species that will accept H+ (protons) in solution, and makes pH higher (NH3 will accept H+ to form NH4+)

Typically, acid/bases are shown in the left (reactants)

when we write the products:

Bronsted Lowery conjugate base = the base formed when the B.L. acid donates its H+ proton ( i.e. HCl -> Cl-

Bronsted Lowery conjugate acid = the acid formed when the B.L. base accept its H+ proton ( i.e. NH4+ has accept H+ proton)

CH3CHOHCOOH(aq) + H2O(l) <-> CH3CHOHCOO-(aq) + H3O+(aq)

acid = CH3CHOHCOOH, since it donates H+

base= H2O, since it accepts H+

conugate base = CH3CHOHCOO-(aq) (can accept H+)

conjugate acid = H3O+(aq) (can donate H+)

Q2.

ratio at

pH = 3.86

then

this can be clacualted using buffer equation

pH = pKa + log(CH3CHOHCOO-/CH3CHOHCOOH)

pH = 3.86 = log((CH3CHOHCOO-/CH3CHOHCOOH))

3.86 = 3.86 + log(CH3CHOHCOO-/CH3CHOHCOOH)

(CH3CHOHCOO-/CH3CHOHCOOH) = 10^(3.86-3.86)

(CH3CHOHCOO-/CH3CHOHCOOH) = 10^0

CH3CHOHCOO-/CH3CHOHCOOH) = 1

CH3CHOHCOO- = CH3CHOHCOOH