General Chemistry II, PAL Worksheet – Week 4 (Wed/Th)

PAL – Week 4 (Wed/Th) #6 Review of Ka, Kb, Polyprotics, LA/LB, Buffers (15.8, 15.9-15.11, 16.1-16.3) Name: Part A: More Acid Base Chem: Calculating Ka and Kb 1. The pH of a 1.00 M solution of urea, which is a weak monoprotic base is 7.050. Determine the Ka for the protonated form or urea. 2. A 0.505 g sample of a monoprotic base (Molar Mass = 45.09 g/mol) was dissolved in water to produce 100.0 mL of solution with a pH = 11.84. What is the ionization constant of this base? 1 Part B: Polyprotic Acids True or False ______A polyprotic acid is an acid with only 1 ionizable H ______ H2SO3 is a polyprotic acid 2- ______ SO3 is a polyprotic base - _______C2H3O2 is a polyprotic base ______ When calculating the pH for a polyprotic acid or base solution, in most cases, one only needs to consider the first ionization step. 3. Consider the ionization of sulfurous acid a) Write the ionization steps associated with the ionization reactions. Label them with Ka1 and Ka2. -4 -8 b) The Ka1 for the first step is 1.6x10 and the second step is Ka2 = 6.4x10 . Why the huge difference? c) Calculate the pH of a 1.0 M solution of sulfurous acid. 2 Part C: Lewis Acid/Base Definitions 1. a) What is the definition of a Lewis acid? b) What is the definition of a Lewis base? 2. Label the following compounds as a Lewis acid or Lewis base. Hint: Draw the Lewis structure. a) PH3 b) SiF4 c) H2S d) BCl3 e) amphetamine 3. Complete the following Lewis Acid-Lewis Base reactions below and label the Lewis acid and Lewis base. a) Al(OH)3 (s) + OH - (aq) à - b) BeCl2 (aq) + 2Cl (aq) à c) Mn 2+ - (aq) + 6CN (aq) à d) CO2(g) - + OH (aq) à e) Copper (II) nitrate and ammonia (hint: reaction from Experiment 1) 3 Part D: Understanding Buffers Conceptually-Come back to this next time if you need to. True or False and Discussion A buffer resists pH change by neutralizing added strong acid or added strong base. - A buffer must contain significant amounts of 1) a weak acid (HA) and it’s conjugate base (A ) or 2) a weak base (A ) and it’s conjugate acid (HA). An effective buffer has relative concentrations of HA and A- that differ by no more than a factor of 10. A buffer [HA] and [A-] = 0.50M is more effective than a buffer with [HA] and [A-] = 0.050M. Why? ___________________________________________________________________________ ________________________________________________________________________________ A buffer can absorb an unlimited amount of base. If False, why?_____________________________________________________________________ ________________________________________________________________________________ - -4 A solution of HNO2 and NO2 , with a Ka = 4.5 × 10 would be best to buffer an aqueous solution at a pH of 4.5. If False, why?_____________________________________________________________________ ________________________________________________________________________________ ________________________________________________________________________________ 4. I) II) III) IV) V) Which of the following can act as buffer solutions? 0.1 M HC2H3O2/0.1 M NaC2H3O2 0.1 M NH3/0.1 M NH4Cl 0.1 M HNO3/0.1 M NaNO3 0.1 M H2SO3/0.1 M NaHSO3 0.1 M KHSO4/ 0.1 M H2SO4 Explain your choices ________________________________________________________________________________ ________________________________________________________________________________ ________________________________________________________________________________ ________________________________________________________________________________ 4