5 Steps to a 5 AP Chemistry 2019

290 ❯ STEP 4. Review the Knowledge You Need to Score High

sure to record your initial volume. You will add small amounts of base drop by drop until

the indicator changes color. Record the final volume. The final volume reading minus the

initial volume reading is the volume of base added.

Calculations

The calculation of the concentration of the base is essentially a stoichiometry reaction.

Most of the time you will be able to generalize the process using the equation:

H+ + OH- → H 2 O

From the molarity of the base and the volume used, you can calculate the moles of base

(OH-). Because of the 1:1 stoichiometry that also will be the moles of acid. Dividing that

by the liters of acid solution pipetted into the flask gives the acid’s molarity.

Comments

This type of titration can be performed with a pH meter without an indicator. The pH

readings will be plotted against the volume. The endpoint is the point of inflection of the

curve. A titration, either with an indicator or a pH meter, can be used to determine the

acid content of household substances such as fruit juices or sodas.

Experiment 5: Chromatography

Synopsis

Many times the components (solutes) in a solution cannot be separated by simple physical

means. This is especially true of polar solutes because of their interactions. One method

that is commonly used is chromatography (Figure 19.3). A very small amount of the solu-

tion is spotted onto a strip of filter paper or chromatography paper and allowed to dry.

The strip is placed vertically into a jar containing a small amount of solvent. As the solvent

Acid (or base)

and indicator

Stopcock

Buret containing

base (or acid)

Figure 19.2 General acid–base titration setup.