Titration

• strong acids ionize almost completely

• weak acids don’t ionize very much• [H[H33OO+1+1] ] notnot same as acid same as acid

concentrationconcentration

[H3O+1]

• can’t be measured directly

• determined by comparison

Acid-Base Titration

• carefully controlled neutralization rxncarefully controlled neutralization rxn• requires:requires:

– standard solutionstandard solution & – acid-base indicatoracid-base indicator

• standard solution is: acid/base of knownknown concentration

TitrationTitration

• standard solution slowly added to unknown solution

• as solutions mix:– neutralization reaction occurs

• eventually: – enough standard solution is added to

neutralize the unknown solution equivalence pointequivalence point

Equivalence pointEquivalence point

total # moles H+1 ions donated by acid =

total # moles H+1 accepted by base

so: total moles H+1 = total moles OH-1

TitrationTitration

• end-pointend-point: point at which indicator changes color

– if indicator chosen correctly: • end-point very close to equivalence point

Titration: strong acid with strong base

volume of 0.100 M NaOH added (ml)

pH

0-

14-

7- equivalence pt

phenolphthalein phenolphthalein color change: color change: 8.28.2 to 10 to 10

0 ml

40ml

20 ml

MMH+1H+1VVH+1H+1 = M = MOH-1OH-1VVOH-1OH-1

• MH+1 = molarity of H+1

• MOH-1 = molarity of OH-1

• VH+1 = volume of H+1

• VOH-1 = volume of OH-1

MaVa = MbVb

• [true for:– monoprotic acids with monohydroxy bases

– diprotic acids with dihydroxy bases

– triprotic acids with trihydroxy bases]

IF:

# H’s in acid

(Ma)(Va) = (Mb)(Vb)

≠ # OH’s in base

need to modify equation:

(#H’s) (#OH’s)

Titration Problem #1

In a titration of 40.0 mL40.0 mL of a nitric acid solution, the end point was reached when 35.0mL35.0mL of 0.100M0.100M NaOHNaOH was added.

calculate the concentration of the calculate the concentration of the nitric acid solutionnitric acid solution

Neutralization Reaction

HNO3 + NaOH H2O + NaNO3

•HNO3 is a monoprotic acid

•NaOH is a monohydroxy base

Variables

• Ma = ?

• Va = 40.0 mL

• Mb = 0.100 M

• Vb = 35.0 mL

• #H’s = #OH’s = 1

Plug and Chug

(1)(x) (40.0 mL) = (0.100 M )(35.0mL)(1)

X = 0.875 M HNO3

Titration Problem #2

• What is theWhat is the concentration concentration of aof a hydrochloric acid solutionhydrochloric acid solution if 50.0 mL50.0 mL of a 0.250M KOH0.250M KOH solution is needed to neutralize 20.0mL 20.0mL of anof an HCl HCl solutionsolution of unknown concentration?

Neutralization Reaction

KOH + HCl H2O + KCl

•HNO3 is a monoprotic acid

•KOH is a monohydroxy base

Variables

• Ma = X

• Va = 20.0 mL

• Mb = 0.250 M

• Vb = 50.0 mL

• #H’s = #OH’s = 1

Plug and Chug

(1)(X)(20.0 mL) = (0.250 M) (50.0 mL)(1)

X = 0.625 M HCl

Titration Problem #3

• What is theWhat is the concentration concentration of aof a sulfuric acid solutionsulfuric acid solution if 50.0mL 50.0mL of aof a 0.25 M KOH0.25 M KOH solution is needed to neutralize 20.0mL 20.0mL of theof the H H22SOSO44 solutionsolution of unknown concentration?

Neutralization Reaction

• H2SO4 + 2 KOH 2 H2O + K2SO4

• H2SO4 is a diprotic acid

• KOH is a monohydroxy base

Variables

• Ma = X

• Va = 20.0mL

• Mb = 0.25M

• Vb = 50.0mL

• #H’s = 2

• #OH’s = 1

Plug and Chug

(2)(X)(20.0ml) = (0.25M)(50.0ml)(1)

X = 0.3125 M H2SO4(sulfuric acid)