In many common acids you have used in laboratory work, all hydrogens are acidic. This is true for acids such as sulfuric acid, H₂SO₄, nitric acid, HNO3, and hydrochloric acid, HCl. In this laboratory activity, you will learn that only some hydrogens within selected acid molecules may be capable of forming H⁺.

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To determine experimentally the number of ionizable H⁺ in an unknown solid acid and to form hypotheses regarding relations between bonding and acidity.

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Wear protective glasses and an apron at all times. Avoid skin contact with solids and solutions. Wash any spills copiously with water. Dispose of all solutions in the container designated by your teacher. Wash your hands before leaving the laboratory.

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Prepare a data table to record laboratory data. This data table must provide space for the molecular formula of the unknown solid acid, the mass of the vial and acid before and after each titration, the molarity of the standardized base, and the initial and final volumes of the sodium hydroxide, NaOH, for each of the three titrations.

1. Obtain a vial of the unknown solid acid from your teacher. Record the molecular formula from the label.
2. Obtain and clean a buret. Rinse and fill the buret with base solution (0.250 M sodium hydroxide, NaOH). Record the initial buret reading.
3. Determine mass of the vial to the nearest 0.01 g.
4. Transfer some solid acid into a clean flask until the vial's mass is decreased by 0.50 to 0.80 g. Record the mass of vial and acid remaining.
5. Add 50 mL of distilled water to the flask to dissolve the sample. Add 3 drops of phenolphthalein.
6. Slowly add base from the buret to the acid solution with constant swirling of the flask until a faint pink color persists for at least 30 s. Insure all the solid acid has dissolved.
7. Do not overshoot the equivalence point. If you add too much base, add a little more of the solid acid, reweigh the vial, and continue the titration.
8. Repeat the titration with two more samples of the unknown solid acid. Use knowledge gained during the first titration to estimate the total volume of base needed.
9. Clean all equipment and thoroughly wash and dry the laboratory bench top.
10. Wash hands thoroughly before leaving the laboratory.

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1. For each titration, calculate:
   1. a) the number of moles of solid acid used.
   2. b) the number of moles of OH^- used.
   3. c) the number of moles of H⁺ used.
2. Determine the average ratio of moles of hydrogen ions to moles of acid. This value will give you the number of ionizable hydrogens in each acid molecule.
3. Rewrite the molecular formula from the vial to reflect the number of ionizable hydrogens.

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1. Sketch a particulate model of your acid, using "*" for each ionizable hydrogen and "~" for the remainder of the acid molecule.
2. Draw a Lewis-dot or line sketch consistent with the modified formula for your acid.
3. Develop an hypothesis to account for your experimental results.

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Teachers Guide

Preparing for the Laboratory Activity

• Major Chemical Concept
• Level
• Expected Student Background
• Time
• Safety
• Materials
• Advance Preparation

Conducting the Laboratory Activity

• Pre-Lab Discussion
• Teacher/Student Interaction
• Anticipated Student Results
• Answers to Data Analysis
• Answers to Imply, Apply
• Post-Lab Discussion
• Possible Extension

Assessing the Laboratory Learning

• Pencil/Paper Items

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Preparing for the Laboratory Activity

Major Chemical Concept

It is easier to remove hydrogens attached to oxygen from an acid molecule than to remove hydrogens attached to carbon or other central atoms.

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Advanced chemistry

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Students should be able to:

• Clean and use a buret
• Explain the process of titration
• Explain "equivalence point"
• Determine the mass of sample to proper precision
• Apply the Arrhenius definition of acids and bases
• Distinguish between molecular structural formulas

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50 min

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• NaOH and some of the acids are irritants to the skin and eyes. Tell the students to use them with caution.
• If you decide to prepare your own sodium hydroxide solution, expect the heat to be released to be rapid with the possibility of splattering. An ice bath for the mixing vessel is useful.

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Non-Consumables (per lab team)

• 1 Balance, 0.01 g sensitivity
• 1 Buret, 50 mL
• 3 Erlenmeyer flasks, 250 mL
• 1 vial, small, for solid acid unknown

Consumables

• Solid acid unknown, 3 g (See list of unknown candidates below.)
• 0.250 M NaOH, 200 mL, 10.0 g per 1.00 L solution

Labels on the vials should contain only the molecular formulas of the acids used. You may also wish to code each unknown with a number for quick reference.

Solid Acids From Which To Choose:

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1. Each student or team needs a vial with about 3.00 g of each of the solid acids. Use at least two different acids in one class.
2. Labeled acid vials should be stored for reuse.

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Conducting the Laboratory Activity

Pre-Lab Discussion

During the pre-lab discussion draw the molecular and structural formula for ethanoic (acetic) acid on the overhead or chalkboard. Point out that where each hydrogen is and to which species it is bonded. Be certain to point out the two types of hydrogens in the molecule. Say no more at this time.

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You should be available during the laboratory session to assist students regarding what the object of this particular activity really is, but it is best to let students do most of the thinking and processing of data. Silent support is sometimes the best type of support during the activity.

See the questions under Imply, Apply in the student part of this activity. You may want to use one or more of these questions or make up your own. Sample problems should be used after the laboratory activity.

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Sample: Benzoic Acid

1. 1. (0.50 g) x (1 mol/122 g) = .0041 mol Benzoic acid.
   2. (0.164 mL) x (0.250 M) = .0041 mol OH^- used.
   3. mol H⁺ used = mol OH^- used = .0041 mol H⁺.
2. mol H⁺ acid/mol acid = 0.0041 mol/0.0041 mol = 1:1
3. Benzoic acid, HC₇H₅O₂; citric acid, H₃C₆H₅O₆; fumaric acid, H₂C₄H₂O₄; maleic acid, H₂C₄H₂O₄; oxalic acid, H₂C₂O₄•2H₂0.

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1. Benzoic acid *~; citric ***~; fumaric, maleic and oxalic acids **~
2. Diagrams of acids (see grids A & B)
3. Hydrogen attached to hydroxyl group are bonded less tightly than those bonded to the carbon.

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Make it clear to students that this was not a titration lab as such nor was it an acid-base activity, but rather a bonding investigation. The key concept is that it is easy to pull off hydrogens attached to carboxylic oxygens and difficult to remove hydrogens attached to carbon or other central atoms. Furthermore, these differences were determined at the lab bench. In fact, bonding affects acidity a great deal.

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Create a laboratory activity to determine the number of moles of ionizable (acidic) hydrogens per mole of H₃PO₃, phosphorous acid. Draw the Lewis-dot diagram for H₃PO₃ and predict the ratio of ionizable hydrogens (acidic) per mole of phosphorous acid, H₃PO₃. Test your hypothesis.

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Assessing the Laboratory Learning

Two grids are designed to evaluate this laboratory activity. Use Grid A if bonding and acidity are stressed in the evaluation; use Grid B if bonding, acidity, and nomenclature are stressed. You may want to use both of these as is or create a composite evaluation that uses parts from each. It is recommended that Grid A and Grid B not both be used together, because some of the supplied questions are answered by looking at the combined grids. The key factor is not to evaluate what has not been taught. The answer to each grid question is in parenthesis after the question.

Pencil/Paper Items

Consider the Figure in answering these questions. Some answers will include more than one species. Some choices may be used once, some more than once, and some may not be used at all in answering the following questions.

1. Which pair are isomers? (A and C)
2. Which contain two ionizable hydrogens per molecule? (A, B, & C)
3. Which contain one ionizable hydrogen per molecule? (D, F, G, and H)
4. What is the total number of acids in Grid A? (8)
5. Which acid is capable of losing three ionizable hydrogens per molecule? (E)
6. Which acid has a molar mass of 60.0 g/mol? (G)
7. Which species containing OH group(s) do not have ionizable hydrogens? (E, F, H, and I)
8. Which pair of acids contains two carbon atoms per molecule? (B and G)
9. How many ionizable hydrogens are there in I? (None)
10. How many ionizable hydrogen are there in F? (One)

(For Use If Bonding, Acidity and Nomenclature Are Stressed)

Consider the second Figure in answering these questions. Some answers will include more than one species. Some choices may be used once, some more than once, and some may not be used at all in answering these questions.

Note: The same ten questions for the first figure should be used for the second as well. Again make use of either Figure, but not both unless you make up your own questions.

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