Acid/Base Titration• Technique used to determine the

concentration of an acid or base by comparison with a standard.

• A neutralization reaction is carried out.

• Rely on color change to signal end of reaction.

Titration: Monoprotic Acid + Monohydroxy Base

• HA + MOH MA + H2O• Use the relationship in the

balanced equation to find the unknown concentration.

• At the end-point, # moles HA = # moles MOH

How to calculate # of moles?

• Remember Molarity!

• Molarity = # moles solute

• Rearranging: # moles solute =

Liters of solution

Molarity X Liters of solution

Acid-Base Titration

• Carefully controlled neutralization reaction.

• Requires a standard solutionstandard solution and an acid-base indicatoracid-base indicator.

• Standard solution has an acid or base of known concentration.

TitrationTitration• Standard solution is slowly added to

unknown solution.• As the solutions mix, a neutralization

reaction occurs.• Eventually, enough standard solution is

added to neutralize the unknown solution. Equivalence pointEquivalence point.

Simple Titration Animation

Equivalence pointEquivalence point

• Total number of moles of H+ ions donated by acid = total number of moles of H+ accepted by base.

• Total moles H+ = total moles OH-

TitrationTitration

• End-pointEnd-point = point at which indicator changes color.

• If the indicator is chosen correctly, the end-point is very close to the equivalence point.

Titration of a strong acid with a 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 Between pH of 4 and 10, only

a few dropsdrops of base are added.

0 ml

40ml

20 ml

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

• MH+ = molarity of H+

• MOH- = molarity of OH-

• VH+ = volume of H+

• VOH- = volume of OH-

• If you know 3, you can find the 4th.

MaVa = MbVb

• True for monoprotic acids and monohydroxy bases.

Titration Problem #1

• In a titration of 40.0 mL40.0 mL of a nitric acid solution, the end point is reached when 35.0 mL35.0 mL of 0.100 M 0.100 M NaOHNaOH is added. Calculate the Calculate the concentration of the nitric acid concentration of the nitric acid solution.solution.

Neutralization Reaction

• HNO3 + NaOH H2O + NaNO3

• 1 H in acid & 1 OH in base.

• MaVa = MbVb

Variables

• Ma = ?

• Va = 40.0 mL

• Mb = 0.100 M

• Vb = 35.0 mL

Plug and Chug

• X (40.0 mL) = (0.100 M )(35.0 mL)

• X = .0875 M

Titration Problem #2

• What is the concentration of a What is the concentration of a hydrochloric acid solutionhydrochloric acid solution if 50.0 50.0 mLmL of a 0.250 M KOH0.250 M KOH solution are needed to neutralize 20.0 mL of the 20.0 mL of the HCl solutionHCl solution of unknown concentration?

Neutralization Reaction

• KOH + HCl H2O + KCl

• 1 H in acid & 1 OH in base

• MaVa = MbVb

Variables

• Ma = X

• Va = 20.0 mL

• Mb = 0.250 M

• Vb = 50.0 mL

Plug and Chug

• X (20.0 mL) = (0.250 M) (50.0 mL)

• X = 0.625 M

Titration Problem #3

• What is the concentration of a What is the concentration of a sulfuric acid solutionsulfuric acid solution if 50. mL of a 50. mL of a 0.25 M Ba(OH)0.25 M Ba(OH)22 solution are needed to neutralize 20 mL of the 20 mL of the HH22SOSO44 solution solution of unknown concentration?

Neutralization Reaction

• H2SO4 + Ba(OH)2 2 H2O + BaSO4

• H2SO4 is a strong diprotic acid.

• Ba(OH)2 is a strong dihydroxy base.• # of H’s in acid = # of OH’s in base.• MaVa = MbVb

Variables

• Ma = X

• Va = 20. mL

• Mb = 0.25 M

• Vb = 50. mL

Plug and Chug

• X (20.) = (0.25 M) (50. mL)• X = 0.625 M = MH+

• To calculate the Molarity of the H2SO4, adjust for the fact that the acid is diprotic.

[H[H++] vs. [H] vs. [H22SOSO44]]

• H2SO4 2H+ + SO42-

• Molarity of H2SO4 = ½ [H+]

• ½ (0.625 M) = 0.31 M H2SO4

Other kinds of Titrations

• Titrations based on redox reactions are also used.

• Same ideas: rely on color change to signal completion of reaction.

• Use stoichiometric equalities to determine concentration of unknown.

Redox titration Animation

Salts• A salt is an ionic compound containing positive ions

other than H+ and negative ions other than OH-

• Hydrolysis is the opposite of neutralization. – Literally means “water splitting.”– A reaction between water and the ions of a dissolved salt.

• When a salt undergoes hydrolysis, it reacts with water to form an acid and a base.

Acid + Base Salt + Water

Hydrolysis

• To predict which solution occurs when salts react with water, i.e., neutral, acidic or basic:– Identify the cation and anion present in the salt– Determine if from a strong or a weak acid/base

• General Rule:strong acid + strong base neutral salt

strong acid + weak base acidic salt weak acid + strong base basic salt

Buffers

• Buffers are solutions that resist changes in pH when limited amounts of acid or base are added.

• A buffer is a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid.

• The mixture of ions and molecules resist changes in pH by reacting with any H+ or OH- added to the solution.