Examples of Strong Acids: Strong Acid Formula Common Source Hydrochloric Acid HCl Stomach Acid. Sulfuric Acid H 2 SO 4

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1 LAB 5. ACIDS AND BASES: PH AND BUFFERS PURPOSE: To determine the ph of common acids and bases using a ph meter, ph paper, and red cabbage indicator. To test the effect of adding an acid or base to a buffer solution. SAFETY CONCERNS: Always wear safety goggles. Wash with soap and water if skin contacts acids or bases. ACIDS: An Acid is a substance that when dissolved in water will produce hydrogen ions, H +, in the solution. An acid that does not contain carbon is called an inorganic acid. An acid that contains carbon is called an organic acid. Strong acids are acids that produce lots of hydrogen ions, H 1+ s. They almost completely dissociate (break apart) to form hydrogen cations, H 1+, and the companion anions. Once dissolved in water the very strong acid does not exist any more since it has dissociated into its ions. Dissociation of Hydrochloric acid H-Cl H 1+ + Cl 1- Examples of Strong Acids: Strong Acid Formula Common Source Hydrochloric Acid HCl Stomach Acid Sulfuric Acid H 2 SO 4 Battery Acid Weak acids are acids that produce only a few hydrogen ions, H 1+ s. Only some of the molecules in solution dissociate (break apart) into hydrogen cations, H 1+, at a time. When they do break apart, the hydrogen cations, H 1+, and the companion anions join back together again to reform the acid. This process of the acid breaking apart and then reforming over and over is called equilibrium. Because of the equilibrium there is a mixture of the original acid, the hydrogen ion, H 1+, and the anion all in the solution at the same time. Dissociation of Acetic acid H-C 2 H 3 O 2 H 1+ + C 2 H 3 O 2 1- Dissociation of Carbonic acid CO 2 + H 2 O H 2 CO 3 H 1+ + HCO 3 1- CH110 Lab 5. Acids and Bases (W14) 51

2 Examples of Weak Acids: Weak Acid Formula Common Source Acetic Acid HC 2 H 3 O 2 Vinegar Carbonic Acid H 2 CO 3 In Carbonated Water Citric Acid H 3 C 6 H 5 O 7 In Lemons & Oranges Tartaric Acid H 2 C 4 H 4 O 6 In Grapes Phosphoric Acid (A weak inorganic acid) H 3 PO 4 In Cola Drinks BASES: A Base is a substance that when dissolved in water will produce hydroxide ions, OH 1-, in the solution. A base that does not contain carbon is called an inorganic base. A base that contains carbon is called an organic base. Strong bases are bases that produce lots of hydroxide ions, OH 1-. They almost completely dissociate (break apart) to form hydroxide anions, OH 1-, and the companion cations. Dissociation of Sodium Hydroxide NaOH Na 1+ + OH 1- Examples of Strong Bases: Strong Base Formula Common Source Sodium Hydroxide NaOH Lye, Caustic Soda, (strong) Soda Ash, Drano Potassium Hydroxide (strong) KOH Potash Weak bases are bases that produce only a few hydroxide ions, OH 1-. Sometimes it s not obvious that hydroxide anions, OH 1-, are even produced, but the anions that are produced can react with water to produce OH 1- s. Weak bases are in equilibrium with their ions. Dissociation of Sodium Bicarbonate NaHCO 3 Na 1+ + HCO 3 1- HCO H 2 O H 2 CO 3 + OH 1-52 CH110 Lab 5. Acids and Bases (W14)

3 Examples of Weak Bases: Weak Base Formula Common Source Magnesium Hydroxide Mg(OH) 2 In Milk of Magnesia Ammonium Hydroxide NH 4 OH In Glass Cleaner Sodium Bicarbonate NaHCO 3 Baking Soda Calcium Carbonate CaCO 3 Antacids, Sea Shells, Egg Shells, Limestone & Marble PH: The ph of a solution is an indicator of the number of hydrogen ions, H 1+ s, present in a solution. We measure the concentration of hydrogen ions, H 1+ s, in moles per liter, M, which we symbolize with square brackets, [H 1+ ]. Pure water contains a small amount of both hydrogen ions, and hydroxide ions. In pure water the concentrations of hydrogen ion and hydroxide ions are the same. [H 1+ ] = [OH 1- ] = 0.000,000,1 M. It is awkward to work with numbers as small as 0.000,000,1 M. It helps to report the hydrogen ion concentration, [H 1+ ], in scientific notation. To simplify reporting the concentration of H 1+ s even more, we use the positive value of the exponent only and call it the ph. (in scientific notation) [H 1+ ] [H 1+ ] = 1x10 -ph (log means opposite of exponent on base 10) ph = -log [H 1+ ] 0.000,1 M 1 x 10-4 M ,000,1 M 1 x 10-7 M ,000,000,1 M 1 x M 10 If the [H 1+ ] = [OH 1- ] then the solution is considered to be neutral and the ph = 7 If the [H 1+ ] > [OH 1- ] then the solution is considered to be acidic and the ph < 7 If the [H 1+ ] < [OH 1- ] then the solution is considered to be basic and the ph > 7 [H 1+ ]= ph = Increasingly Acidic [H 1+ ] > [OH 1- ] Neutral [H 1+ ] = [OH 1- ] Increasingly Basic [H 1+ ] < [OH 1- ] CH110 Lab 5. Acids and Bases (W14) 53

4 The [H 1+ ] will not always be a simple 1 x 10 whole number value. In such cases we can estimate the ph range or we can calculate the ph exactly using the ph equation: ph = -log [H + ]. Examples: [H 1+ ] = 1x10 -ph ph = -log [H 1+ ] Given [H 1+ ] If [H 1+ ] = 0.000,032M = 3.2 x 10-5 M Calculated [H 1+ ] Then [H 1+ ] = 1 x = 2.5 x 10-9 M We can estimate ph to be between 4 & 5 We can estimate [H 1+ ] to be between 10-8 & 10-9 Calculated ph Then ph = -log(3.2x10-5 ) = 4.5 Given ph If ph = 8.6 INDICATORS: The ph of a solution is often measured by observing how the acid or base causes the color of certain organic molecules to change. Litmus, a chemical found in lichen, is one of many acid-base indicators. Litmus turns from blue to red in acidic solutions and from red to blue in basic solutions. Phenolphthalein, a laxative, is colorless when acidic but turns brilliant pink above ph 7. Anthocyanin indicators are common in flower petals, berries, and purple cabbage. The colors change over the entire spectrum of ph making it a universal indicator. The chart below shows the relative color changes of anthocyanin pigments over a wide ph range. Colors of Anthocyanin (Red Cabbage) Indicator ph = Color Red Fushia Rose Purple (neutral) Blue Aqua Green Greenbrown Yellow NEUTRALIZATIONS: A Neutralization reaction is a reaction between a strong acid and a strong base in which the product formed is neither acidic nor basic. For example, if hydrochloric acid (an acid) is mixed with sodium hydroxide (a base), the products will be sodium chloride (table salt) and water. Neutralization: Strong Acid + Strong Base Salt + Water HCl (aq) + NaOH (aq) NaCl (aq) + H 2 O 54 CH110 Lab 5. Acids and Bases (W14)

5 BUFFERS: A Buffer is a solution that will not drastically change in ph even when strong acids or bases are added to it. A buffer is made by combining a weak acid and a weak base in a water solution. By having both an acid and a base in the solution together there will always be an acid to react with any base added, or there will be a base to react with any acid added. For example, a simple buffer could be made by combining equal amounts of the weak acid, Acetic Acid, and the weak base, Sodium Acetate. Buffer = Weak Acid + Weak Base HC 2 H 3 O 2(aq) + NaC 2 H 3 O 2(aq) (weak acid) (weak base) + NaOH If a strong base gets added to the buffer, it reacts with the weak acid present (rather than giving the solution more OH- s) and gets turned into NaC 2 H 3 O 2(aq) a weak base that does not drastically change the ph of the solution. + HCl If a strong acid gets added to the buffer, it reacts with the weak base present (rather than giving the solution more H+ s) and gets turned into HC 2 H 3 O 2(aq) a weak acid that does not drastically change the ph of the solution. As long as the number of OH 1- s and H 1+ s does not drastically change then the ph does not drastically change. Buffer solutions are important in keeping the ph of biological solutions in a narrow range. CH110 Lab 5. Acids and Bases (W14) 55

6 PROCEDURES: ACTIONS: I. DETERMINATION OF PH OF COMMON ITEMS: 1. Arrange your large test tubes and into each one put a different sample of a liquid 1 (about 1 inch deep) or a solid (about a dime sized scoop) of household product to be tested. 2. Add to each sample enough deionized water so that the probe of your ph meter can just be immersed into the liquid. Stir or stopper and shake each tube to dissolve. 3. Use these sample tubes for the sections A, B, & C where the ph of each will be determined by various methods. A. By Meter: 1. Clean the ph electrodes of your ph meter with deionized water between each sample test. 2 Be gentle with the electrodes as they are delicate and can easily break or scratch. 2. Carefully submerge the cleaned electrode of a ph meter into the samples indicated. 3 Read and record the ph on your report sheet to the accuracy of your meter. 3. Calculate the [H 1+ ] from your ph meter reading and record it on your report sheet. B. By Paper: 1. Tear the universal indicator ph paper into 1 cm square pieces and arrange them on a watch glass or paper towel Dip a glass stirring rod into the sample to be tested and touch the wet stirring rod to the ph paper. 3. Compare the color produced on the wet 5 paper to the color on the ph paper container label. Record the results. C. By Cabbage (Anthocyanins): 1. Tear a piece of purple cabbage 6 into small pieces and place in a glass beaker Add deionized water to barely cover the cabbage and then boil it using a hot plate, burner, or microwave oven until the water is dark purple. Remove from heat. 3. Into the test tube or beaker of the sample to be tested, 8 add a few mls of purple cabbage juice and observe the color change. Record the ph range. 9 NOTES: 1 If the liquid is highly viscous, (thick like shampoo) treat it as a solid. 2 We must have clean electrodes for each test. If you do not rinse off the previous sample then the new sample gets contaminated and the ph can be altered. 3 Toilet cleaner and drano/lye are very strong and so tax the flexibility of our meters. To avoid recalibrating our meters we will test these with other methods. 4 Bring the sample to be tested to the ph paper rather than inserting the ph paper into the solution. Putting the ph paper into the solution washes pigment from the paper into the sample. 5 Compare the color when the paper is wet. The color will change as the paper dries and not compare accurately. 6 Roses, hydrangeas, and blueberries also contain anthocyanin pigments and will work as indicators. We use red cabbage because it is so inexpensive and readily available. 7 If working at home use a coffee mug or glass container. Metal pans cause the purple pigments to turn blue. 8 Because tea and coffee are so dark in color it is difficult to see any change in the cabbage pigment. We will rely on other methods to test these. 9 Compare the colors of your substance-cabbage mix with the colors on the chart in the discussion section. This is not exact but just allows you to predict a general ph range. 56 CH110 Lab 5. Acids and Bases (W14)

7 II. PH OF CARBON DIOXIDE, CO 2, SOLUTION: 1. Place about 10 mls of deionized water into a large test tube or small beaker and determine the ph using a ph meter Keeping the ph electrode in the tube, place a straw in the water and blow bubbles into it. 11 Watch the ph meter to observe changes. 3. Continue to blow bubbles into the water for a couple of minutes. Record any ph changes on the report sheet. III. BUFFERS: 1. Obtain 4 large test tubes and label them Tubes A-D. Into both tubes A and B place about 10 mls of deionized or distilled water. Into both tubes C and D make buffer solutions by combining 5 mls of 0.1 M Sodium Acetate, (NaC 2 H 3 O 2 ) with 5 mls of 0.1 M 0.1 M Acetic acid, (HC 2 H 3 O 2 ) and mix well. 10 The deionized water may not be at a ph of 7 depending on the amount of CO 2 gases already dissolved in it. Record the ph as you find it and use this as your starting point. 11 As you blow into the tube you are adding Carbon Dioxide, CO 2 to the water. CO 2 + H 2 O forms H 2 CO 3. Review the disassociation of Carbonic acid, H 2 CO 3, in the laboratory discussion of weak acids. A. Water with Acid: 2. Measure and record the ph of the water in tube A using a meter Add 1 drop of 1.0 M HCl, Hydrochloric Acid, and swirl the sample to mix. Record the ph. 4. Continue to mix in 1 drop of HCl at a time, recording the ph after each drop until a total of 5 drops have been added. Discard the solution in the sink. B. Water with Base: 5. Measure and record the ph of the water using a meter Add 1 drop of 1.0 M NaOH, Sodium Hydroxide, and swirl the sample to mix. Record the ph. 7. Continue to mix in 1 drop of NaOH at a time, recording the ph after each drop until a total of 5 drops have been added. Discard the solution in the sink. 8. Analyze the results and report your conclusions on the report sheet. CH110 Lab 5. Acids and Bases (W14) 57

8 C. Buffer with Acid: 9. Measure and record the ph of the buffer solution in tube C using a meter Add 1 drop of 1.0 M HCl, Hydrochloric Acid, and swirl the sample to mix. Record the ph. 12 Not all buffers are neutral. The ph depends on the weak acid and the weak base used. Each buffer will have its own unique ph. Record the ph of yours and use this as your starting value. 11. Continue to mix in 1 drop of HCl at a time, mixing and recording the ph after each drop until a total of 5 drops have been added. Discard the solution in the sink. D. Buffer with Base: 12. Measure and record the ph of the buffer solution in tube D using a meter Add 1 drop of 1.0 M NaOH, Sodium Hydroxide, and swirl the sample to mix. Record the ph. 14. Continue to mix in 1 drop of NaOH at a time, recording the ph after each drop until a total of 5 drops have been added. Discard the solution in the sink. 15. Analyze the results and report your conclusions on the report sheet. 58 CH110 Lab 5. Acids and Bases (W14)

9 LAB 5. ACIDS AND BASES: PRE LAB EXERCISES: NAME DATE A solution has a ph of 8.7. The solution is A. Acidic B. Basic C. Neutral Which solution has the smaller hydrogen ion concentration? A. Solution A which has a hydrogen ion, [H 1+ ], concentration of 3.8 x B. Solution B which has a hydrogen ion, [H 1+ ], concentration of 2.3 x C. Not enough information to predict. Which solution has the lower ph? A. Solution A which has a hydrogen ion, [H 1+ ], concentration of 3.8 x B. Solution B which has a hydrogen ion, [H 1+ ], concentration of 2.3 x C. Not enough information to predict. Which solution has the higher ph? A. 0.1 M HCl. B. 0.1 M HC 2 H 3 O 2 C. Neither as both solutions have the same ph. Sodium hydroxide turns purple cabbage to a yellow color. Ammonium hydroxide turns purple cabbage green. Which solution has the lower ph? A. 0.1 M NaOH. B. 0.1 M NH 4 OH C. Neither as both solutions have the same ph. A buffer is A. a solution of acid and base that should always have a ph of 7. B. formed when a strong acid neutralizes a strong base. C. a solution of a weak acid and a weak base that resists change in ph. D. all of these. 7. Write the equation of Hydrochloric acid (HCl) with sodium acetate (NaC 2 H 3 O 2 ) in a buffer solution. 8. Write the equation of Sodium Hydroxide (NaOH) with acetic acid (HC 2 H 3 O 2 ) in a buffer solution. CH110 Lab 5. Acids and Bases (W14) 59

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11 LAB 5. ACIDS AND BASES: REPORT: NAME PARTNER DATE I: Determination of ph by Meter, Paper, and Cabbage: Solution A. ph by Meter Water (for comparison): Deionized water used Foods: Soda Pop (light color) brand Tea or Coffee (circle which) Orange Juice Vinegar [H 1+ ] = 10 -ph B. ph by Paper C. New Cabbage color produced ph by Cabbage OMIT as color is too dark to see a difference with cabbage Cleaners: Shampoo Detergent Ammonia Bleach Toilet Cleaner Lye Others: Baking Soda (NaHCO 3) Milk of Magnesia (MgOH) Antacid brand Do not insert the meter into this solution Do not insert the meter into this solution Results Summary: 1. In general, foods and beverages are mostly A. Acidic B. Basic C. Neutral D. there is no general trend 2. In general, cleaning supplies are mostly A. Acidic B. Basic C. Neutral D. there is no general trend Conclusion/Explanation/Analysis: Why were the results as they were? Explain any anomalies. 3. Show the reaction with stomach acid that explains how a Tums antacid (CaCO 3 ) settles an upset stomach and gives off gas that make you burp. CH110 Lab 5. Acids and Bases (W14) 61

12 II. ph of Carbon Dioxide, CO 2, in water: ph of Deionized Water = ph of Breath Carbonated Water = Complete and balance the equation for the reaction of carbon dioxide with water to form carbonic acid. CO 2 + H 2 O Conclusion/Explanation of Results: (Why did the CO 2 from your breath have the effect that it did?) Results Summary: 1. The ph of deionized water may not be exactly 7.00 because (Based on what should have occurred above.) A. there is dissolved carbon dioxide gas in the water that forms carbonic acid. B. there are dissolved hard water ion impurities in the water. C. water is naturally acidic due to the 1x10-7 M of H 1+ ions present. D. all of these. 2. Which of the following is true about the solubility of substances in water? (Based on what should have occurred above and the principles of gas solubility.) A. Increasing the temperature has no effect on the quantity of a gas that will dissolve in water. B. Increasing the temperature will usually decrease the quantity of a gas that will dissolve in water. C. Increasing the temperature will usually increase the quantity of a gas that will dissolve in water. D. There is no general trend regarding temperature and gas solubility. 3. A sample of deionized water had a ph of 6.0 until it was boiled. Once this water was boiled and cooled it had a ph of 7? Explain how this could happen. (Based on principles of gas solubility.) 4. Bacteria in the mouth converts sugar into acids that are capable of dissolving tooth enamel. Repeated drinking of carbonated beverages, even those that do not contain sugar, can also contribute to loss of enamel. Explain why carbonated beverages can act in this way and cause dental caries (tooth decay). 62 CH110 Lab 5. Acids and Bases (W14)

13 III. Buffers: Solution: ph Solution: ph A. Water with Acid C. Buffer with Acid Deionized Water After 1 drop 1.0M HCl After 2 drops 1.0M HCl After 3 drops 1.0M HCl After 4 drops 1.0M HCl After 5 drops 1.0M HCl NaC 2 H 3 O 2 :HC 2 H 3 O 2 Buffer After 1 drop 1.0M HCl After 2 drops 1.0M HCl After 3 drops 1.0M HCl After 4 drops 1.0M HCl After 5 drops 1.0M HCl B. Water with Base D. Buffer with Base Deionized Water After 1 drop 1.0M NaOH After 2 drops 1.0M NaOH After 3 drops 1.0M NaOH After 4 drops 1.0M NaOH After 5 drops 1.0M NaOH Conclusion/Explanation/Analysis: (Why were the results as they were? When adding HCl or NaOH to water, what caused the ph to change or not change as it did? Explain the chemistry that was taking place. Explain any anomalies.) NaC 2 H 3 O 2 :HC 2 H 3 O 2 Buffer After 1 drop 1.0M NaOH After 2 drops 1.0M NaOH After 3 drops 1.0M NaOH After 4 drops 1.0M NaOH After 5 drops 1.0M NaOH Conclusion/Explanation/Analysis: (How does the buffer resist ph change?) Results Summary: 1. In general, adding a strong acid or a strong base to water A. neutralizes the solution making a solution that is neither acidic nor basic. B. drastically changes the ph to be either strongly acidic if strong acid is added, or strongly basic if strong base is added. C. causes only a slight change in ph making the solution a only a little more acidic than it already was if strong acid is added, or only a little more basic than it already was if strong base is added. D. has variable results as there is no general trend 2. In general, adding a strong acid or a strong base to a buffer solution A. neutralizes the solution making a solution that is neither acidic nor basic. B. drastically changes the ph to be either strongly acidic if strong acid is added, or strongly basic if strong base is added. C. causes only a slight change in ph making the solution a only a little more acidic than it already was if strong acid is added, or only a little more basic than it already was if strong base is added. D. has variable results as there is no general trend CH110 Lab 5. Acids and Bases (W14) 63

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