Name: _____________________ Lab Day/Time: _____________________ Lab Group Number: ____________________ CHM 116 Prelab Activity for Week 7 Investigation 40: ?What Is the Acid Dissociation Constant? Read Section 17.3 (up to page 737) in your text, paying attention to the following items: Definition of equivalence point General characteristics of titration pH curves Comparison between strong acid-strong base and weak acid-strong base titration curves. In this investigation, your group will determine the molarity and Ka value for an unknown mono-protic weak acid by titrating the weak acid with a strong base (NaOH) solution. Use the sample weak acid titration curve shown below and accompanying Titration Data to answer the questions below. Show your work and write answers in spaces provided. At the equivalence point in an acid-base titration: Total moles of the weak acid (HA) = Total moles of added hydroxide, OH? . If 28.2 mL of a 0.100 M NaOH solution was needed to reach the equivalence point, how many moles of the weak acid, HA, were in the original 50.0 mL? total moles HA = ___________ When a strong base (OH?) is added to a weak acid, the OH? converts (neutralizes) the weak acid, HA, to H2O and the conjugate base, A?, as shown in the equation below. HA(aq) + OH? ? H2O(l) + A?(aq) In the sample titration, how many moles of HA were neutralized, i.e., converted to H2O and A?, when only 10.0mL of the 0.100 M NaOH had been added to the original HA solution. moles HA neutralized = _________ Based on the answers from (b) and (a), how many moles of HA remain in the 60.0 mL solution (50.0 mL weak acid + 10.0 mL NaOH)? moles HA remaining = ( a ? b ) = _________ How many moles of the conjugate base, A?, are in the 60.0 mL solution? Hint: Look at your answer to (b) ? this should be the same number of moles of HA neutralized. moles A? = _________ OK, now it?s time to convert moles to concentration (molarity). What are the concentrations of HA and A? , [HA] and [A?], based on your answers from (c) and (d). Remember that the total volume of the solution is now 60.0 mL. [HA] = ___________ [A?] = ___________ Next piece to the puzzle. When 10.0 mL of NaOH had been added to the original 50.0 mL of weak acid solution, the pH of the resulting 60 mL was 4.75 (refer to the titration curve). Based on this pH value, what is the hydronium ion concentration? [H+] = ___________ And finally, based on the equilibrium of a weak acid, HA ? H+ + A? , write the expression for Ka and use the values obtained in (e) and (f) to calculate the Ka value for the sample weak acid. Ka = ___________ 4. The previous questions give an example of how the Ka value for a weak acid can be determined using titration data. If presented with the pH curve for a weak acid-strong base titration, there is another method to determine the Ka value for the weak acid. This alternative is called the ?half-equivalence point? method. Refer to the site HYPERLINK "http://icn2.umeche.maine.edu/genchemlabs/Equil_Const/equilcon1.html" http://icn2.umeche.maine.edu/genchemlabs/Equil_Const/equilcon1.html to learn about this method. After reading this information, determine the Ka value for an unknown weak acid given the following titration data. Volume of base needed to reach the equivalence point = 25.0 mL pH of the titration solution after 12.5 mL of base has been added = 4.74 Ka for the unknown weak acid = ____________
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