TitrationSE+(4)

2019 Name: Aylin Jobby, Jyostsana Joy Student Exploration: Titration 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? The paper turns red How does litmus paper indicate a neutral substance? The strips stay the same color How does litmus paper indicate a base? The paper turns 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? The solution turns blue, which indicates the pH is now above 7.0

2019 Activity A: Acids and bases Get the Gizmo ready :  Click Reset . Select 1.0 M HNO 3 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 (HNO 3 ) dissociates into H + and NO 3 – 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 HNO 3 are dissolved in a liter of water, you would say [HNO 3 ] = 0.6 M. A. Because HNO 3 is a strong acid , it dissociates almost completely in water. That means the concentration of H + is very nearly equal to that of HNO 3 . What is [H + ] if [HNO 3 ] 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.) 2 C. What is the pH of a 0.6 M HNO 3 solution? 0.22 2. Describe : The equation for the reaction of nitric acid (HNO 3 ) and sodium hydroxide (NaOH) is shown on the bottom right of the Gizmo. A. What are the reactants in this reaction ? Nitric acid and sodium hydroxide B. What are the products of this reaction? NaNO3, H2O 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 HNO 3 into the flask until the phenolphthalein begins to lose its color. Stop adding HNO 3 when the color change is permanent. A. How much (HNO 3 ) was required to cause the indicator to change color? 8.9 mL B. What can you say about the pH before and after the last drop of HNO 3 was added? Before the last drop, It was above 8.2 and after the last drop it was above 8.2 (Activity A continued on next page)

2019 Activity A (continued from previous page) 4. Explore : Click Reset and change the indicator to Bromthymol blue . Add exactly 8.8 mL of HNO 3 to the flask. A. What does the color of the indicator tell you about the current pH of the flask? The indicator is blue, which means the pH is above 7.6 B. Add one more drop of HNO 3 . What does the color tell you about the pH now? The color turns yellow, which means the pH is now 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 HNO 3 ? The last drop of HNO3 caused the pH to change from above 8.2 to below 6.0 5. Apply : Water has a pH of 7. If 0.1 mL (about one drop) of 1.0 M HNO 3 is added to 100 mL of water, the result is a solution with a concentration of 0.001 M HNO 3 . A. What is the pH of 0.001 M HNO 3 ? 3 B. How much did one drop of HNO 3 cause the pH of water to change? From 7 to 3 C. How does this relate to what you determined in question 4C? It shows that HNO3 can lower the pH a lot. 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? The titration curve rises at the equivalence point when the pH increases. Why do you think the titration curve has the shape it has ? The pH rises sharply as the acids neutralizes and the additional bases are left over. Volume titrant (mL)