Wear goggles when working with chemicals

Determination of Ka for a On Labs, Inc. Version 42-0151-00-02

Review the safety materials and wear goggles when working with chemicals. Read the entire exercise before you begin. Take time to organize the materials you will need and set aside a safe work space in which to complete the exercise.

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Experiment Summary:

You will learn about and the interactions that occur between a weak acid and a strong base. You will also learn the equations that allow the Ka to be determined from a titration curve and will then perform a to find the Ka for an unknown weak acid. You will identify the weak acid by comparing the experimental Ka to a variety of known weak acid Ka values.


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Learning Objectives Upon completion of this laboratory, you will be able to:

Identify the defining characteristics of acids and bases.

Define acidity, and explain how the to the acidity of a substance.

Distinguish between strong and weak acids in relationship to the acid ionization constant (Ka).

Discuss the purpose of a pH titration curve, and define the half-equivalence point and equivalence point.

Perform pH analysis utilizing pH indicator strips.

Perform a titration of a weak acid against a strong base.

Construct an accurate titration curve using experimental results.

Determine Ka for an unknown weak acid utilizing the data collected during the experiment.

Time Allocation: 2.5 hours

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Experiment Determination of Ka for a Weak Acid

Materials Student Supplied Materials

Quantity Item Description 1 Bottle of distilled water 1 Dish soap 1 Drinking glass or cup 1 Pair of scissors 1 Roll of paper towels 1 Sheet of white paper 1 Source of tap water

HOL Supplied Materials

Quantity Item Description 1 Glass beaker, 100 mL 1 Graduated cylinder, 10 mL 1 Pair of gloves 1 Pair of safety goggles 1 pH test strips, wide range 1 Experiment Bag: Determination of Ka for a Weak Acid

1 – Phenolphthalein solution, 1% – 0.5 mL in pipet 1 – Pipet, empty short stem 1 – Sodium hydroxide, 1 M – 30 mL in dropper bottle 1 – Unknown #108 – 25 mL in dropper bottle

Note: To fully and accurately complete all lab exercises, you will need access to:

1. A computer to upload digital camera images.

2. Basic photo editing software, such as Microsoft Word or PowerPoint, to add labels, leader lines, or text to digital photos.

3. Subject-specific textbook or appropriate reference resources from lecture content or other suggested resources.

Note: The packaging and/or materials in this LabPaq kit may differ slightly from that which is listed above. For an exact listing of materials, refer to the Contents List included in your LabPaq kit.

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Experiment Determination of Ka for a Weak Acid

Background Acids and Bases

Scientists categorize substances as either acids or bases depending on their properties and the way they interact with other chemicals. Classification of an acid or a base is useful when predicting how a chemical will react with another chemical. For example, researchers must consider acid- base reactions when designing antacids to help treat heartburn or when creating cleaning products. Because acids are proton donors and bases are proton acceptors, the interaction of an acid and a base will result in a chemical reaction that is called neutralization.

The generalized definition of an acid is as a substance that has a sour taste and will react with a base. Strong acids are corrosive to metals and will cause severe burns on contact with skin. While strong acids, such as concentrated sulfuric a

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