ACIDS AND BASES

Acid Base Titration

A very accurate method to measure concentration.

Acid + Base Salt + Water

H+ + OH- H2O

Moles H+ = Moles OH-

Buret

Solution with Indicator

Acid-Base Titrations The amount of acid or base in a solution

is determined by carrying out a neutralization reaction; an appropriate acid-base indicator (changes color in specific pH range) must be used to show when the neutralization is complete.

This process is called a TITRATION: the addition of a known amount of solution to determine the volume or concentration of another solution.

3 steps… Add a measured amount of an acid

of unknown concentration to a flask.

Add an appropriate indicator to the flask (i.e. phenolphthalein)

Add measured amounts of a base of known concentration using a buret. Continue until the indicator shows that neutralization has occurred. This is called the end point of the titration.

Example: A 25-mL solution of H2SO4 is neutralized by 18 mL of 1.0 M NaOH using phenolphthalein as an indicator. What is the concentraion of the

H2SO4 solution?

Equation: 2NaOH + H2SO4 2H2O + Na2SO4

How many mol of NaOH are needed for neutralization?

NaOH mol 018.0L 1

mol 1 NaOH L 0.018

Example: A 25-mL solution of H2SO4 is neutralized by 18 mL of 1.0 M NaOH using phenolphthalein as an indicator. What is the concentraion of the

H2SO4 solution?

Equation: 2NaOH + H2SO4 2H2O + Na2SO4

How many moles of H2SO4 were neutralized?

4242 SOH mol 0090.0

NaOH mol 2

SOH mol 1 NaOH mol 0.018

Example: A 25-mL solution of H2SO4 is neutralized by 18 mL of 1.0 M NaOH using phenolphthalein as an indicator. What is the concentraion of the

H2SO4 solution?

Equation: 2NaOH + H2SO4 2H2O + Na2SO4

Calculate the concentration of the acid:

L

mol M

M 0.36 solution L 0.025

SOH mol 0.0090 M 42

Titration Curve: a graph showing how the pH changes as a function of the amount of added titrant in a titration.

Data for the graph is obtained by titrating a solution an measuring the pH after every drop of added titrant.

Titration Curve: a graph showing how the pH changes as a function of the amount of added titrant in a titration.

Equivalence point: the point on the curve where the moles of acid equal the moles of base; the midpoint of the steepest part of the curve (the most abrupt change in pH) is a good approximation of the equivalence point.

Titration Curve: a graph showing how the pH changes as a function of the amount of added titrant in a titration.

Knowledge of the equivalence point can then be used to choose a suitable indicator for a given titration; the indicator must change color at a pH that corresponds to the equivalence point.

Calculations of Titrations

1) The Mole Method of Molarity:

Calculate the molarity of a sulfuric acid solution if 23.2 mL of it reacts with 0.212 g of Na2CO3.

H2SO4 + Na2CO3 Na2SO4 + CO2 + H2O

32

42

32

3232 CONa mol 1

SOH 1mol

CONa 106.0g

CONa 1molCONa 0.212g

42SOH mol 0.00200

M 0.0862 L 0232.0

mol 0.00200

L

mol M

2) Normality:

The normality (N) of a solution is defined as the (M) x (total positive oxidation)

oxidation positive totalMolarityNormality

Examples:

HCl H+ + Cl-

H2SO4 2H+ + SO42-

NaOH Na+ + OH-

Ba(OH)2 Ba2+ + 2OH-

+ ox = 1

+ ox = 2

+ ox = 1

+ ox = 2

Example: Calculate the molarity and normality of a solution that contains 34.2 g of Ba(OH)2 in 8.00 L of solution.

mol 6199.0g 171.3

mol 1Ba(OH) g 34.2 2

M 5002.0L 00.8

mol 0.1996M

N 0005.02L 1

mol 0.0250N

In titration problems, you can use this equation:

base) base)(vol. (norm. acid) acid)(vol. (norm.

VNVN bbaa

Example: 30.0 mL of 0.0750 N HNO3 required 22.5 mL of Ca(OH)2 for neutralization. Calculate the normality and molarity of the Ca(OH)2 solution.

N 0.100N

mL) )(22.5(NmL) N)(30 (0.0750

VNVN

b

b

bbaa

M 0.0500M

)2(MN100.0

ox)(MN