1 (355)

Prévia do material em texto

Chapter 17 Aqueous Ionic Equilibrium 355 17.49 (a) Given: 500.0 mL pure water Find: initial pH and pH after adding 0.010 mol HCI Conceptual Plan: Pure water has a pH of 7.00 then mL L then mol HCI, L [H₃O⁺] pH 1L 1000 mL = [H₃O⁺] Solution: Pure water has a pH of 7.00, so initial pH = 7.00, 500.0 mL X 1L = 0.5000 L, then M = mol L = 0.010 0.5000 mol L = 0.020 M Because HCI is a strong acid, it dissociates completely; so pH = -log [H₃O⁻] = -log (0.020) = 1.70. Check: The units (none) are correct. The magnitude of the answers makes physical sense because the pH starts neutral and then drops significantly when acid is added and no buffer is present. (b) Given: 500.0 mL buffer of 0.125 M and 0.115 M NaC₂H₃O₂ Find: initial pH and pH after adding 0.010 mol HCI Other: = 1.8 X 10⁻⁵ Conceptual Plan: Initial pH: Identify acid and base components then M NaC₂H₃O₂ M C₂H₃O₂⁻ then acid = HC₂H₃O₂, base = NaC₂H₃O₂(aq) Na⁺(aq) + C₂H₃O₂⁻(aq) M pH pH after HCI addition: Part I: Stoichiometry: mL L then [NaC₂H₃O₂], L mol NaC₂H₃O₂ and [HC₂H₃O₂], L mol 1L 1000 mL L write balanced equation then HCI + NaC₂H₃O₂ + NaCl mol NaC₂H₃O₂, mol mol HCI mol NaC₂H₃O₂, mol HC₂H₃O₂ then set up stoichiometry table Part II: Equilibrium: mol NaC₂H₃O₂, mol L, pH Solution: Initial pH: Acid = HC₂H₃O₂, so [acid] = [HC₂H₃O₂] = 0.125 M. Base = Because one ion is generated for each NaC₂H₃O₂, = 0.115 M = [base ]. Then = pH after addition: 500.0 mL X 1000 1L = 0.5000 L then 0.125 mol 0.5000 L = 0.0625 mol and 0.115 mol NaC₂H₃O₂ 1L X 0.5000 = 0.0575 mol NaC₂H₃O₂. Set up a table to track changes: HCl(aq) + NaC₂H₃O₂(aq) HC₂H₃O₂(aq) + NaCl(aq) Before addition 0.00 mol 0.0575 mol 0.0625 mol 0.00 mol Addition 0.010 mol After addition 0.00 mol 0.0475 mol 0.0725 mol 0.10 mol Because the amount of HCI is small, there are still significant amounts of both buffer components; so the Henderson-Hasselbalch equation can be used to calculate the new pH. 0.0475 = log (1.8 X + 0.0725 4.56 Check: The units (none) are correct. The magnitude of the answers makes physical sense because the pH started below the pKₐ of the acid and it dropped slightly when acid was added. Copyright © 2017 Pearson Education, Inc.