Titration Gizmo Answer Key

Name: Cason Byrd Date: 06-08-2022 Student Exploration: Titration Directions: Follow the instructions to go through the simulation. Respond to the questions and prompts in the orange boxes. Vocabulary: acid, analyte, base, dissociate, equivalence point, indicator, litmus paper, molarity, neutralize, pH, strong acid, strong base, titrant, titration, titration curve, weak acid, weak base Prior Knowledge Questions (Do these BEFORE using the Gizmo.) There are several definitions of acids and bases. According to the Brønsted-Lowry definition, an acid is a substance that is capable of donating a proton to another substance. A base is a substance that accepts protons. When an acid and a base are combined, the acid is neutralized as the base accepts the protons produced by the acid. One way to determine if a solution is acidic or basic is to use litmus paper, as shown above. There are two types of litmus papers: red and blue. How does litmus paper indicate an acid? Both strips turn red. How does litmus paper indicate a neutral substance? The red strip stays red and the blue strip stays blue. How does litmus paper indicate a base? Both strips turn blue Gizmo Warm-up Litmus is an example of an indicator, a substance that changes color depending on its pH (pH is a measure of the concentration of protons, or H+ ions). In the Titration Gizmo, you will use indicators to show how acids are neutralized by bases, and vice versa. To begin, check that 1.00 M NaOH is selected for the Burette, Mystery HBr is selected for the Flask, and Bromthymol blue is selected for the Indicator. 1. Look at the flask. What is the color of the bromthymol blue indicator? Yellow 2. What does this tell you about the pH of the solution in the flask? pH is below 6.0 Solutions with a pH below 7.0 are acidic, while those with a pH above 7.0 are basic. 3. Move the slider on the burette to the top to add about 25 mL of NaOH to the flask. What happens, and what does this tell you about the pH of the flask? When the solution turns blue, the pH has risen above 7.6. Get the Gizmo ready: Activity A: Acids and bases ● Click Reset. Select 1.0 M HNO3 for the Burette and Mystery NaOH for the Flask. ● Select Phenolphthalein for the Indicator. ● You will need a scientific calculator for this activity. Introduction: When most acids dissolve in water, they dissociate into ions. For example, nitric acid (HNO3) dissociates into H+ and NO3– ions. Question: How do acids and bases interact in solution? 1. Calculate: Concentration is measured by molarity (M), or moles per liter. Brackets are also used to symbolize molarity. For example, if 0.6 moles of HNO3 are dissolved in a liter of water, you would say [HNO3] = 0.6 M. A. Because HNO3 is a strong acid, it dissociates almost completely in water. That means the concentration of H+ is very nearly equal to that of HNO3. What is [H+] if [HNO3] is 0.01 M? 0.01 M B. The pH of a solution is equal to the negative log of H+ concentration: pH = –log[H+] What is the pH of this solution? (Use the “log” button on your calculator.) C. What is the pH of a 0.6 M HNO3 solution? 2 0.22 2. Describe: The equation for the reaction of nitric acid (HNO3) and sodium hydroxide (NaOH) is shown on the bottom right of the Gizmo. A. What are the reactants in this reaction? B. What are the products of this reaction? 3. Measure: A titration can be used to determine the concentration of an acid or base by measuring the amount of a solution with a known concentration, called the titrant, which reacts completely with a solution of unknown concentration, called the analyte. The point at which this occurs is called the equivalence point. Carefully add HNO3 into the flask until the phenolphthalein begins to lose its color. Stop adding HNO3 when the color change is permanent. A. How much (HNO3) was required to cause the indicator to change color? 8.9mL B. What can you say about the pH before and after the last drop of HNO3 was added? The pH was higher than 8.2 prior to the last dip. The pH was lower than 8.2 after the final dip. 4. Explore: Click Reset and change the indicator to Bromthymol blue. Add exactly 8.8 mL of HNO3 to the flask. A. What does the color of the indicator tell you about the current pH of the flask? Since it is blue, the pH is higher than 7.6. B. Add one more drop of HNO3. What does the color tell you about the pH now? The yellow hue indicates a pH level below 6.0. C. If you combine the results of this question with the results from question 3B, what do you know about the total pH change caused by adding the last 0.1 mL of HNO3? The pH dropped from over 8.2 to below 6.0 as a result of the final drop of HNO3. 5. Apply: Water has a pH of 7. If 0.1 mL (about one drop) of 1.0 M HNO3 is added to 100 mL of water, the result is a solution with a concentration of 0.001 M HNO3. A. What is the pH of 0.001 M HNO3? 3 B. How much did one drop of HNO3 cause the pH of water to change? from 7 to 3 C. How does this relate to what you determined in question 4C? This demonstrates how much a single drop of reduce a solution's pH. 6. Explain: A titration curve is a graph of pH vs. volume of titrant. The graph at right shows a typical titration curve for the titration of a strong acid by a strong base. (A strong base is one that has relatively high dissociation in water.) A. How would you describe the shape of the titration curve? Initial titrant addition results in a modest pH increase. The pH rises quite sharply in the vicinity of the equivalence point. B. Why do you think the titration curve has the shape it has? can The pH stays low even after an acid and a base react, because there is still acid in the mixture. Any extra base that remains after the acid has been neutralized will cause the pH to quickly climb. Activity B: Determining concentration Get the Gizmo ready: ● Click Reset. Select 1.00 M NaOH for the Burette and Mystery H2SO4 for the Flask. ● Select Bromthymol blue for the Indicator. Introduction: Adding a drop of strong acid or base into a neutralized solution is similar to adding a drop of strong acid or base to water—it causes an abrupt change in pH. By using an appropriate indicator, a chemist can tell when a solution is neutralized by monitoring its color. Question: How is titration used to determine an unknown concentration? 1. Measure: Titrate the sulfuric acid analyte (H2SO4) with the sodium hydroxide titrant (NaOH). How much 1.00 M NaOH is needed to neutralize the H2SO4 solution? 21.6 mL 2. Interpret: The balanced equation for the reaction of H2SO4 and NaOH is given at bottom right. Based on this equation, how many moles of NaOH react with 1 mole of H2SO4? 2 3. Manipulate: Recall that molarity is equal to the number of moles of a substance dissolved in one liter of solution: molarity = moles ÷ volume. A. Write an equation for determining the number of moles of NaOH that are added to the flask based on [NaOH] and volume of NaOH titrant (mL NaOH): Moles NaOH = B. Write a similar expression for the number of moles of H2SO4 in the flask based on [H2SO4] and the volume of H2SO4 (mL). Moles H2SO4 = C. Because there are twice as many moles of NaOH as moles of H2SO4 in this reaction, you can say: Moles NaOH = 2 · Moles H2SO4 Substitute your expressions from 3A and 3B into this equation and solve for [H2SO4]: D. Now calculate [H2SO4] based on the data from the Gizmo. [H2SO4] = 0.108 M 4. Calculate: Select the Worksheet tab. This tab helps you calculate the analyte concentration. ● Fill in the first set of boxes (“moles H2SO4” and “moles NaOH”) based on the coefficients in the balanced equation. (If there is no coefficient, the value is 1.) ● Record the appropriate volumes in the “mL NaOH” and “mL H2SO4” boxes. ● Record the concentration of the titrant in the M NaOH box. Click Calculate. What is the concentration listed? 0.108 M Click Check. Is this the correct concentration? Yes If you get an error message, revise your work until you get a correct value. (You may have to redo the titration if you do not have the correct volume of titrant.) 5. Practice: Perform the following titrations and determine the concentrations of the following solutions. In each experiment, list the volume of titrant needed to neutralize the analyte and the indicator used. Use the Worksheet tab of the Gizmo to calculate each analyte concentration. Include all units. Titrant Analyte Indicator Titrant volume Analyte concentration 0.70 M KOH HBr Indicator choice will vary. 30.0 mL 0.210 M 0.50 M HCl Ca(OH)2 Indicator choice will vary. 8.4 mL 0.021 M 0.80 M H2SO4 NaOH Indicator choice will vary. 5.6 mL 0.090 M 6. Apply: Once you know the concentration of a strong acid or a strong base, you can estimate its pH. Use pH = –log10[H+] to calculate the pH of each of the strong acid mystery solutions (Mystery HBr and Mystery H2SO4) based on the concentrations you determined in questions 4 and 5. Check your answers with the Gizmo. (Because dissociation is not always complete, your answers may vary slightly from values in the Gizmo.) [H2SO4] = 0.108 M pH H2SO4 = 0.967 [HBr] = 0.210 M pH HBr = 0.678 7. Apply: For a strong base, the concentration of hydroxide ions [OH–] is estimated to be the same as the concentration of the base. The pH of a strong base is found with the equation pH = 14 + log10[OH–]. Based on their concentrations, find the [OH–] and pH of each of the strong bases. Check your answers with the Gizmo. [NaOH] = 0.090 M [OH-] = -1.05 pH NaOH = 12.95 [Ca(OH)2] = 0.021 M [OH-]* = -1.68 pH Ca(OH)2 = 12.32 *Note: For Ca(OH)2, the OH– concentration is double the Ca(OH)2 concentration because there are two OH– ions in each Ca(OH)2 molecule. Activity C: Weak acids and bases Get the Gizmo ready: ● Click Reset. ● Select 1.00 M NaOH for the Burette and Mystery CH3COOH for the Flask. Introduction: Unlike strong acids and bases, weak acids and weak bases dissociate relatively little in water. Some ions are formed, but the remaining molecules remain whole. As a result, the pH of a weak acid or base is closer to neutral than the pH of a strong acid or base. When weak acids or bases react with strong bases or acids, the resulting salts often act as bases or acids themselves, causing the pH at the equivalence point to vary from 7.0. This can impact your choice of indicator. Question: What happens when weak acids and bases are titrated? 1. Gather data: For each indicator given in the Gizmo, what is the pH range over which it changes color? Bromthymol blue: 6.0-7.6 Methyl orange: 3.1-4.4 Phenolphthalein: 8.2 2. Interpret: The salt produced by the reaction of acetic acid and sodium hydroxide is a weak base, CH3COONa. As a result, the pH of the equivalence point is slightly basic. The titration curve for this reaction is shown below: Volume NaOH A. Why is methyl orange not a good indicator to use for this titration? The pH of 4.4 is the point at which methyl orange changes color. This titration curve shows that the pH increases over 4.4 well before the corresponding point is attained. B. What would be a better indicator to use, and why? A preferable indicator to employ is phenolphthalein because it changes color at a pH of roughly 9, which is close to the equivalency point. 3. Experiment: Perform two titrations, the first using methyl orange as an indicator and the second using phenolphthalein as an indicator. Record the volume required to reach a color change with each indicator: Volume NaOH (methyl orange): 2.0 mL Volume NaOH (phenolphthalein): 24.3 mL A. Why did you get such different results with each indicator? The pH climbs above 5 before the equivalence point is reached, per the titration curve for a weak acid titrated by a strong base, which is shown in the graph. As a result, only a small amount of titrant is needed to change the color of the methyl orange. When the equivalency point is achieved, the hue of the phenolphthalein changes more precisely. B. Which value would you use to calculate the acetic acid concentration, and why? Because the hue of phenolphthalein changes when the equivalency point is reached, I would use it. 4. Infer: The salt produced by the reaction of a weak base and a strong acid is acidic. A. Based on this fact, what can you say about the equivalence point of this reaction? The pH at the equivalency point will be lower than 7.0. B. Which indicator would you use for a titration of a weak base such as NH3? Explain. Because methyl orange changes color at a pH lower than 7.0, I would use that. [Note: Bromthymol blue, which turns yellow at pH levels below 6.0, may also be a suitable option.] 5. Calculate: Use the Gizmo to find the concentration of the Mystery CH3COOH and the Mystery NH3. List the titrant and indicator you used for each titration. Titrant Analyte Indicator Titrant volume Analyte concentration CH3COOH Phenolphthalein 24.3 mL 0.243 M NH3 Bromthymol blue 14.7 mL 0.147 M 6. On your own: If you like, you can continue to practice titration calculations by selecting Random for the Flask. Click New to change the analyte. Record your results in the space provided. [Note: Student findings will differ while using the "Random" setting because the Gizmo's analyte concentrations are randomized.]