Section 16.3

Titrations and Buffers

1. To know how to neutralize acids and bases (titration)

Learning Goal

Section 16.3

Titrations and Buffers

Remember that… (add this to your notes)

Acid + Base Water + Salt

Ex.: HCl + NaOH H2O + NaCl

Acid-Base Reactions (review)

This is a double replacement reaction

Notice that H+ and OH- H2O

This is a neutralization reaction

Section 16.3

Titrations and Buffers

Acid-Base Titrations

Titration – delivering a measured volume of a solution of known concentration into a solution being analyzed determine the concentration of the solution (analyte)

We need to neutralize the solution, therefore…

What should you add to an acid to neutralize it?

What should you add to a base to neutralize it?

BASE

ACID

Section 16.3

Titrations and Buffers

Titrant (standard solution)

Analyte (solution being analyzed)

The titration is

stopped when the

pH = 7

↓

Buret – device used for accurate

measurement of the delivery of a liquid

Stoichiometric point

(equivalence point) –

when just enough titrant

has been added to react

with all of the solution

being analyzed

A pH indicator is added to the analyte so the color changes when the pH

reaches 7(or a pH meter could be used)

How will we know when the

solution has been

neutralized?

Section 16.3

Titrations and Buffers

Acid-Base Titrations

• Titration curve (pH curve) – plot of the data (pH vs volume) for a titration

pH changes quickly

close to the equivalence

point.

Section 16.3

Titrations and Buffers

Calculating the Volume of Titrant Needed in Acid-Base Titrations (using M=n/L)

How much 0.100 M NaOH is needed to titrate 50.0 mL of 0.200 M HNO3?

1. Calculate how many moles of H+ are in the analyte.

0.0500 L HNO3 x 0.200 mol H+ = 1.00 x 10-2 mol H+

1L

2. Calculate the volume of titrant solution that contains the needed number of moles of OH- (same as moles of H+).

V = n/M = 1.00 x 10-2 mol OH- = 1.00 x 10-1 L

0.100 mol/L or 100. mL

Section 16.3

Titrations and Buffers

Calculating Molarities and Volumes in Titrations (using one formula) Remember that Molarity x Volume (L) = n• Moles of H+ must equal moles of OH- for neutralization• Since the acid or base may give off more than one H+ or

OH-, we must use the mole ratio of base/acid:

MaVa (b/a) = MbVb

Ex.: H2SO4 + 2NaOH 2H2O + Na2SO4

Notice that this is a double displacement/replacement reaction.

In most neutralization reactions: acid + base water + salt

Section 16.3

Titrations and Buffers

To understand how the pH in a solution (eg. Blood) is kept constant when acid or base are added (buffers)

Learning Goal

Section 16.3

Titrations and Buffers

B. Buffered Solutions

• Buffered solution – resists a change in its pH when either and acid or a base has been added– Presence of a weak acid and its conjugate base

buffers the solution:

Buffer = weak acid + salt containing conj. base of acid

Section 16.3

Titrations and Buffers

B. Buffered Solutions

Section 16.3

Titrations and Buffers

Example of buffered solution:

HC2H3O2, NaC2H3O2

weak acid salt that contains conjugate base of acid

(C2H3O2-)

• If strong acid is added to the solution, the H+ ions react with the conjugate base and form the weak acid:

Net reaction: H+ + C2H3O2- HC2H3O2

• If strong base is added to the solution, the OH- ions extract the protons from the weak acid to form water:

Net reaction: OH- + HC2H3O2 H2O + C2H3O2-