Acid Base Equilibrium

CHEM 14BL

volumetric analysis is an experimental technique because

it is commonly used to determine the concentration of an unknown solution by measuring the volumes required to react completely with an unknown by using a standard solution

standard solution

a solution with a precisely known concentration

volumetric analysis is an

analytical technique

titrant

the solution inside the buret in a titration

acid base titration is a

volumetric analysis

titration graph

volume of titrant v. pH

strong acid

essentially 100% disassociated in water to produce H3O+ (ka = 10^6)

weak acid

disassociates very slightly in water (ka around 10^-6)

strong base

100% disassociated in water to produce OH-

weak base

disassociates very slightly in water (Kb around 10^-5)

terms “strong” and “weak” are

not related to the concentration of an acid or a base, related to the magnitude of ka or kb (disassociation in water)

strong acids have

large ka or small pka

weak acids have

small ka or large pka

strong bases have

large kb or small kb

weak bases have

small kb or large pkb

how to properly collect the experimental data in lab when performing volumetric analysis

use the pH meter as your reference, record buret volume when the pH readings change

approaching the equivalence point

very little NaOH is needed to increase the pH of the solution

equivalence point region

when the pH jumps up

equivalence point region on graph

around mid-point of the “vertical region

titration between a strong acid and a strong base

equivalence point pH = 7

titration between a weak acid and a strong base

pH > 7 (basic)

titration between a weak base and a strong acid

pH < 7 (acidic)

equilibrium exists

at every single point on a titration graph

buffer solution

consist of a weak acid and its conjugate base (or a weak base with its conjugate acid) to resist a change in pH when a strong acid or strong base is added to the buffer solution

if a buffer solution contains both HA and A- and is reacted with a strong base solution

stabilization of the base occurs when HA (weak acid) is in excess so it can neutralize all the base so it can produce the conjugate base

if a buffer solution contains both HA and A- and is reacted with a strong acid solution

stabilization of the strong acid occurs when the conjugate base neutralizes it and produces the conjugate acid to stabilize the pH

buffer pH equatio

what is special about the half-equivalence point

half equivalence point (added half of the titrant) is still within the buffer region

at half equivalence point

[conjugate base] = [weak acid] for the best general purpose buffer

limitations of the henderson-hasselbach equation

ka for the weak acid in buffer < 10^-5 and applies only to buffer solutions

half equivalence point

pH = pka