CHM142 Exam 3 Review 2024/2025 With Correct Answers
Given the pKa's of several polyprotic acids choose the strongest acid among the ions given below. H3PO4: pKa1=2.16, pKa2=7.19, pKa3=12.32 H3AsO4: pKa1=2.26, pKa2=6.77, pKa3=11.29 H2SO3: pKa1=1.77, pKa2=7.21 H2CO3: pKa1=6.37, pKa2=10.33 > HCO3- > H2AsO4- > H2PO4- > HPO42- > HSO3- - Ans: H2AsO4- Which of the following is the weakest base? > (CH3CH2)2NH (pKb = 3.07) > NH2OH (pKb = 7.97) > (CH3CH2)3N (pKb = 3.25) > CH3CH2NH2 (pKb = 3.25) > (CH3)3N (pKb = 4.19) - Ans: NH2OH (pKb = 7.97) Which of the following ions have negligible tendency to accept protons in water? Instructions for making up a buffer solution say to mix 90.0 mL of a 0.125 M solution of acetic acid, CH3COOH, with 20.0 mL of a 0.175 M solution of sodium acetate, CH3COONa. What is the pH of this solution? Ka for acetic acid is 1.8 x 10-5. - Ans: 4.24 What is the pH of the buffer solution which contains 0.50 M ammonia (NH3) and 0.25 M ammonium chloride (NH4Cl) after the addition of 20.0 mL of a 0.20 M solution of HCl to 80.0 mL of the buffer? The pKb for ammonia is equal to 4.74. - Ans: 9.44 The values of pKa's for H3AsO4 are pKa1=2.19, pKa2=6.94, and pKa3=11.50. Use the appropriate pKa to find the value of pKb for HAsO42-. - Ans: 7.06 Consider 0.010 M aqueous solutions of five weak bases. Which solution will have the highest pH?
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