5 Steps to a 5 AP Chemistry 2019

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

Equipment

Test tubes

Stirring rods

Spatula

Assorted glassware

Measurements

This experiment involves no measurements, only estimations of volumes and masses.

Calculations

This experiment involves no calculations.

Comments

Be very careful when working with concentrated ammonia and hydrochloric acid. Always

wear goggles, gloves, and an apron, and keep these reagents in the hood.

Experiment 14: Acid–Base Titrations

Synopsis

Experiments that fall into this category are acid–base titrations involving weak acids or weak

bases. Many times the course of the titration is followed by a pH meter and the equivalence

point is determined graphically. This allows you to determine not only the concentration of the

weak acid or base but also its pKa or pKb. Both monoprotic and polyprotic acids may be exam-

ined. From an examination of the specific reaction involving a weak acid or base, you should be

able to determine whether the solution at the equivalence point will be acidic or basic.

Equipment

Stirring rods

pH meters or pH probes

Buret

Assorted glassware

Measurements

You will be making pH measurements and plotting them against volume of titrant added.

In many cases, you will titrate various acids (strong and weak) with a NaOH solution of

known concentration. The equivalence point for such a titration is the point at which a

dramatic increase in pH occurs; this is called the point of inflection of the curve. The pH at

the volume corresponding to half the equivalence point volume is the pKa of the acid. The

same is true of bases, except the pH will be decreasing during the titration. A polyprotic

acid or base will give you two points of inflection, and two pKs and Ks may be calculated.

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, use Kb = 10 - pKb.

Comments

Be extremely careful when working with the acids and bases and wear all of your personal

protective equipment, especially your goggles. When making dilutions, always add the acid

(or base) to water, NOT water to acid.