5 Steps to a 5 AP Chemistry 2019

Experimental Investigations ❮ 299

Experiment 15: Buffer pH

Synopsis

A buffer is a substance that resists a change in pH when an acid or base is added to

it. It is normally a mixture of a weak acid and its conjugate base. Experiments in this

category involve examining the properties of buffers and household substances that are

buffers. This will involve titrating a substance with an acid or base while following the

course of the titration with a pH meter, plotting the pH versus mL of titrant added,

and determining the equivalence point graphically. At any point before the equivalence

point, you have a buffer present. Common household substances may be tested for their

buffer ability. The curve of pH versus mL of a substance that has some buffering ability

rises sharply initially and then levels off much more than a titration of a substance that

is not a buffer. You can use this to determine whether an unknown solution exhibits

any buffering capability.

Equipment

pH meter

Burets and clamps

Magnetic stirrer

Assorted glassware

Measurements

You will be making measurements of volume and pH for a wide variety of substances. The

point in a titration involving a buffer that corresponds to halfway to the equivalence point

is called the point of maximum buffering.

Calculations

The Ka of the acid can be calculated by the equation Ka = 10 - pKa. If the Kb of a weak base

is to be determined, then use Kb = 10 - pKb.

Comments

Be careful in handling the acid and base solutions.

Experiment 16: The Capacity of a Buffer

Synopsis

Experiments in this category are designed to explore the capacity of a buffer, which is the

amount of acid or base that can be neutralized by the buffer. You can determine this by

using different amounts of the conjugate acid and base components or by changing the

concentration of each by the same amount. Normally, the higher the concentration of the

conjugate acid and base in the buffer, the more moles of added base or acid can be neutral-

ized and thus the higher the buffer capacity. You will also be asked to create a buffer of a

specific pH.

Equipment

Balance

Burets and clamps

Assorted glassware