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Investigation 13: Acid-Base Equilibria

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Quantitative Analysis of Acid Rain

Performance Assessment

In this lab experience, students examine three acid samples and compare and contrast their observed pH with the amount of base needed to reach the endpoint in a titration. Students will apply scientific reasoning and successfully identify their unknowns by considering if the samples are weak or strong and mono or diprotic.

Materials Included in Kit

Consumable:
Adipic acid, 20 g
Nitric acid, 50 mL
Phenolphthalein indicator solution, 100 mL
Sodium hydroxide solution, NaOH, 0.005 M, 100 mL
Sulfuric acid, 20 mL
Additional Materials Required
Beakers, Borosilicate Glass, 50-mL, 30
Buret, Borosilicate Glass, with Glass Stopcock, 50-mL, 10
Cylinder, Borosilicate Glass, 50 mL, 10
Flask, Erlenmeyer, Borosilicate Glass, 250 mL, 30
Flinn pH Meter, 10
Single Buret Clamp, Plastic-Coated Jaw, 10
Support Stand, Economy Choice, 10

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Measure Acid Strength

In this lab experience, students prepare equimolar solutions of weak acids and measure their pH values to rank the acids in order of increasing strength. Students come to understand that the weak acid with the lowest pH is relatively the strongest acid, and the position of its equilibrium lies farthest to the right. Students come to understand that the weak acid with the lowest pH is relatively the strongest acid, and the position of its equilibrium lies farthest to the right.

Materials Included in Kit

Consumable:
Phenolphthalein solution, 0.5%, 30 mL
Potassium dihydrogen phosphate (potassium phosphate, monobasic), 8 g
Potassium hydrogen sulfate (potassium bisulfate), 8 g
Potassium hydrogen phthalate, 8 g
Potassium hydrogen tartrate (potassium bitartrate), 8 g
Sodium hydroxide, 0.1 M, 250 mL
Pipets, 40
Weighing dishes, 20
Additional Materials Required
Beakers, Borosilicate Glass, 150 mL, 10
Bottles, Washing, Polyethylene, 250-mL, 10
Cylinder, Borosilicate Glass, 50 mL, 10
Flask, Erlenmeyer, Borosilicate Glass, 125 mL, 10
Flask, Volumetric, Borosilicate Glass, 100 mL, 10
Flinn pH Meter, 10
Flinn Scientific Electronic Balance, 410 x 0.01-g, 10
Stirring Rods, Glass, 10

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Design a Natural pH Indicator

Engineering Design Challenge

In this lab experience, students are challenged to extract pigments from natural resources to create and test pH indicators. Students will find that natural indicators provide great substitutions for synthetic indicators. The key to creating an effective indicator is picking a natural substance that contains the organic pigment anthocyanin, which changes color as pH changes. Anthocyanins are generally found in flowers and vegetables that are red or purple in color. The pigment is then concentrated by different extraction methods depending on the source. Students will be impressed how well their natural indicators work when measured against a synthetic one. This lab will give them an appreciation of how the chemistry they learn in the classroom applies to the world around them.

Materials Included in Kit

Consumable:
Buffer capsules to prepare standard acid and base solutions of known pH (pH 2–12)
Bromthymol blue, 0.04%, 35 mL
Methyl orange, 0.1%, 35 mL
Thymol blue, 0.04%, 35 mL
Cranberry apple tea, 4 bags
Grape juice, 100%, 5.5-oz can
Hibiscus, 20 g
Rose petals, 20 g
Acetic acid, CH3COOH, 100 mL
Sodium bicarbonate, NaHCO3, 100 mL
Sodium carbonate, Na2CO3, 100 mL
Sodium phosphate, monobasic, 100 mL
Pipets, 80
Additional Materials Required
Beakers, Borosilicate Glass, 100-mL, 40
Beakers, Borosilicate Glass, 150-mL, 30
DLAB Classic Magnetic Stirrer/Hot Plate, 10
Mortar and Pestle Set, Porcelain, Flinn, 10
Reaction Plates, 24 Wells, 20

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Titrations - The Study of Acid-Base Chemistry

In this lab experience, students standardize a sodium hydroxide solution and perform a weak acid–strong base titration to determine the concentration of an unknown vinegar sample. Titration is a very important technique in chemistry and is used more in the everyday world than students realize. This lab gives them a chance to learn and practice that technique. It also shows them that even when you prepare a solution, it can be important to verify its exact concentration. Sometimes even little discrepancies can make a difference in how an experiment is run and what conclusions are reached. But it also shows the limitations of certain techniques and what information can be learned with them. The titration technique is one of many essential tools for chemists and can help students appreciate the need for precision and the variety of data that can be obtained with it.

Materials Included in Kit

Consumable:
Acetic Acid Solution, 750 mL, 1.0 M
Acetic Acid Solution, 750 mL, 1.6 M
Acetic Acid Solution, 750 mL, 0.833 M
Hydrochloric acid, 0.5 M, 60 mL
Phenolphthalein indicator solution, 1.0%, 30 mL
Potassium hydrogen phthalate, 25 g
Sodium hydroxide, 0.1 M, 1 L
Sodium hydroxide solution, 1.0 M, 800 mL
Weighing dishes, 10
Additional Materials Required
Beakers, Borosilicate Glass, 50-mL, 10
Beaker Tongs with protective sleeves, 10
Buret, Borosilicate Glass, with Glass Stopcock, 50-mL, 10
DLAB Classic Magnetic Stirrer/Hot Plate, 10
Flinn Scientific Electronic Balance, 410 x 0.01-g, 10
Single Buret Clamp, Plastic-Coated Jaw, 10
Cylinder, Borosilicate Glass, 100 mL, 10
Flask, Erlenmeyer, Borosilicate Glass, 250 mL, 30
Reaction Plates, 24 Wells, 10
Bottles, Washing, Polyethylene, 250-mL, 10
Funnel, Utility, Polyethylene, 2¾", 10

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Analysis of Buffer Solutions and Ranges

In this lab experience, students observe the effect of adding concentrated acid and base to different buffer solutions. The lab demonstrates how a buffer works and shows the different pH ranges they work in depending on the components of which they are made. Buffers can be a difficult topic for students to understand. Even with explaining to students that buffers are all around them and that their body pH is regulated by such a system, it can be hard to impart the importance of such a system. This lab helps students see how buffers can resist changes in pH. Color changing indicators are a great way for students to observe chemistry happening. They will see that the buffers with the best pH resistance are when the components are in a one-to-one ratio, equal moles of acetic acid and sodium acetate in this case. The higher the concentration of buffer components, the better the overall buffering capacity. A properly made buffer can even resist pH changes when concentrated acids or bases are added. Students will also come to understand that buffers are not completely resistant. Eventually they will reach their buffering capacity and any additional amount of acid or base added will have a large effect on pH.

Materials Included in Kit

Consumable:
Acetic acid solution, 0.1 M, 1 L
Bromocresol green indicator solution, 0.04%, 75 mL
Congo red indicator, 0.1%, 150 mL
Hydrochloric acid, 6 M, 500 mL
Sodium acetate solution, 0.1 M, 1 L
Sodium hydroxide solution, NaOH, 6 M, 500 mL
pH paper, narrow range, 3.0–5.5, roll
Toothpicks, box of 250
Additional Materials Required
Beral Pipets, Graduated, Pkg. of 500, 1
Cylinder, Borosilicate Glass, 10 mL, 20
Reaction Plates, 24 Wells, 40
Test Tubes without Rims, Borosilicate Glass, 13 x 100 mm, 9.0 mL, 20
Test Tubes without Rims, Borosilicate Glass, 16 x 150 mm, 20.0 mL, 140
Test Tube Rack, Economy Choice, 10