chemistry homework

Lab 6: Acids, Bases, and pH Measurements

Lab Materials:

Small plastic cups

Cream of tartar

Tide TM

powder detergent

Red cabbage—may need to look at several grocery stores for this

Water

Sandwich bags

Medicine droppers

Wax Paper

Plastic spoons

Vinegar

Salt

Sugar

Baking soda

Baking powder

Crushed white antacid tablet

Rubbing alcohol

Calculator that will perform log functions

Acids, Bases, and pH Measurements

Name: _____________________________ Score: _________

Introduction 1

If you look around your kitchen, you will find several examples of everyday chemicals called acids

and bases. Vinegar and lemon juice are acids and ammonia and bleach are bases. The fact that these

foods are acidic while the cleaners are basic is typical. Environmentally, acids and bases are critically

important. For example, we’ve all seen how acid rain can damage buildings and lakes.

The strength of acid and base solutions are frequently determined using a pH scale where pH = -

log[H + ]. The pH scale is based on the concentration of hydrogen ions (H

+ or H3O

+ ) in solution and

ranges from 0 (extremely acidic) to 14 (extremely basic) with the midpoint 7 representing neutral

water. If you have ever monitored the water in an aquarium or a swimming pool, then you probably

measured the pH using a colored indicator solution. Indicators are organic compounds that react as

acids or bases (exchanging hydrogen ions in solution) and will change color depending on the pH of

the solution. One can also use a pH meter to measure pH directly.

In this lab, you will use a colored indicator (flavin, an anthocyanin) found in red cabbage to determine

the approximate pH of several substances. Below is a chart demonstrating the color change the

cabbage juice turns in solutions of different pH. 2

pH Indicator Color

2 Red

4 Purple

6 Violet

8 Blue

10 Teal

12 Greenish-yellow

Directions

1. Calculate the pH or the [H + ] for the following solutions. Then, classify them as either acidic,

basic, or neutral. Please note that you will need a scientific calculator for performing log functions.

a. pH = 6.4 b. [H + ] = 4.93 x 10

-10 M

c. [OH - ] = 5.20 x 10

-2 M d. pH = 11.5

2. Preparing the Indicator Solution: (Note: The indicator can stain surfaces.)

Using a small Dixie cup as your measuring device, pour a cup of water into a sandwich bag. Next,

add about 15-20 pieces of cabbage to the bag. You do not want the pieces to be too small. Aim for

pieces that are just slightly larger than a quarter. After sealing the bag (air out), rub the cabbage

together. The best way to describe what you need to do is to pretend you are washing clothes and

scrubbing the dirt out. Continue to extract the pigment until you get a deep dark purple solution.

The pre-lab video illustrates the color intensity you must get. Typically, it takes about 5 minutes.

Next, pour only the liquid into a plastic cup, and throw the cabbage in the trash.

3. Next, you will test the pH of the following substances using the indicator solution you prepared in

step 2. You may use wax paper as your test surface for solids and small plastic cups for the liquids.

The pre-lab video shows the amounts you need of each chemical. When recording your

observations, record the initial color you observe upon addition of the cabbage juice. Then, using

the information in the lab’s introduction, approximate the pH of the substances and classify them as

either acidic, basic, or neutral. 3

4. Choose 3 substances that have different pH values. Calculate the [H + ] for each of them. Show your

work.

Substance Observations Approximate pH Acidic, Basic, or Neutral

Vinegar

Sugar

Cream of Tartar

Baking Soda

Baking Powder

Water

Salt

Crushed antacid tablets

Tide TM

powder

detergent

Rubbing alcohol

5. Pick two acidic substances that gave similar results with the indicator color. Design an experiment that would determine which one of these substances is the stronger acid. Email your

instructor the procedure you have developed prior to actually mixing any chemicals. If you have

not received approval from your instructor by the deadline stated in the syllabus, you may

NOT complete this question.

a. Detailed procedure for testing acid strength

b. Which substance is the stronger acid? Explain.

References: 1 Written with Dr. George Whitwell, Associate Professor of Chemistry at NCWC

2 Helmenstine, A.M., http://chemistry.about.com/od/acidsbase1/a/red-cabbage-ph-indicator.htm,

accessed Dec. 8, 2009. 3 Kessler, James, & Galvan, Patricia. (2004). Inquiry in Action: Investigating Matter Through

Inquiry 2 nd

ed., American Chemical Society.