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Titration POGIL Activity Objectives After complete this activity, students will be able to: 1) Calculate the pH of the solution during any stage of titration. 2) Construct a titration curve . 3) Understand why titration curves have particular shapes 4) Identify the information present in a titration curve Previous knowledge 1) Limiting reagents 2) Weak acid-base equilibria 3) Buffer solutions Consider this The titration curve of weak base with strong acid involves four regions.  Before the addition of strong acid, the solution has only a weak base.  After the addition of some strong acid, but before the equivalence point, some of the weak base has been converted to its conjugate acid. The solution is a buffer.  At the equivalence point, all of the weak base has been converted to its conjugate acid. The solution is a weak acid.  After the equivalence point, the strong acid is in excess. One can ignore the proton form weak acid dissociation. Treat the solution as a strong acid solution. The titration curve of weak acid with strong base involves four regions.  Before the addition of strong base, the solution has only a weak acid.  After the addition of some strong base, but before the equivalence point, some of the weak acid has been converted to its conjugate base. The solution is a buffer.  At the equivalence point, all of the weak acid has been converted to its conjugate base. The solution is a weak base.  After the equivalence point, the strong base is in excess. One can ignore the OH- form weak base hydrolysis. Treat the solution as a strong base solution

Key Question You are titrating 100.0 ml of 0.0500 M ammonia with 0.100 M HCl. Calculate the pH at various stage of titration. Instruction: I. Before performing any calculation, discuss as a group to identify the chemicals involved in the system.  What is ammonia? Is it an acid or base? Strong or weak? Monoprotic or polyprotic?  What is HCl? What reaction will occur between ammonia and HCl? II. Calculating the number of moles of ammonia and HCl. Discuss the following questions as a group.  Compare the amount of ammonia and HCl. Which one is limiting reagent? Which one is in excess?  After the reaction is complete, what species are present in the solution? Is the final solution an acid? A buffer? A base?  After identifying the final species in the end of the reaction, do you know how to calculate the pH of the final solution? III. Calculate the pH of the solution under the following conditions. 1. Before adding any HCl Reaction Initial Change Equilibria

2. After the addition of 10.0 ml of HCl Reaction Initial Change Final

3. After the addition of 25.0 ml HCl Reaction

4. After the addition of 49.5 ml HCl Reaction

5. After the addition of 50.0 ml of HCl

6. After the addition of 50.5 ml HCl

7. After the addition of 60.0 ml HCl

8. After the addition of 75.0 ml HCl

IV. Enter the data in Table 1.1 Volume of HCl added (ml) Calculated pH 0.0 10.0 25. 0 49.5 50.0 50.5 60.0 75.0 V. Use the above data and construct a titration curve (pH verse Volume of titrant).

Draw conclusions: Look at the above titration curve and answer the following questions. 1. What general features does this titration curve have? Which part is the sharpest?

2. At equivalence point (moles of ammonia = moles of HCl), what’s the pH of the solution? Is it basic or acidic? Does it make sense to you?

3. At half way to the equivalence point (V HCl = Vep/2), what’s the relationship between pH and pKa?

4. How do you find out the pKa of an unknown acid from a titration curve?

Applications: 1. A monoprotic base , B, was titrated by a strong acid, HCl. The mass of B is 0.2055 g. a. It took 43.20 ml of 0.04653M HCl solution to reach the equivalence point. What’s the molar mass of this base?

b. At the equivalence point, the pH is 3.26 . Is this base strong or weak?

c. After the addition of 21.60 ml of HCl, the pH is 5.76. What is the pKb value of this base?

2. Calculate the pH of 25.0 ml 0.200 M acetic acid titrated with 0.100 M of KOH solution at the added KOH volume of a) 15.00 ml b) at equivalence point

Titration POGIL 2015.pdf

After the addition of some strong acid, but before the equivalence point, some of the weak. base has been converted to its conjugate acid. The solution is a buffer ...

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