Equilíbrios Aquosos e Tampões

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17 Additional Aspects of Solutions to Exercises Aqueous Equilibria 0.5692 = [HCOONa] [HCOOH] = [0.10 (0.10 M M y)/1L ; 0.5692[0.10(1 - = 0.10 y; 0.05692 = 0.15692 y; y = 0.3627 = 0.36 L 360 mL of 0.10 M HCOONa, 640 mL of 0.10 M HCOOH. Check. The pH of the buffer is less than for the conjugate acid, indicating that the amount of CA in the buffer is greater than the amount of CB. This agrees with our result. 17.32 The solutes listed contain three possible conjugate acid/conjugate base (CA/CB) pairs. These are: HCOOH/HCOONa, = 3.74 = 4.74 HCN/NaCN, = 9.31 For maximum buffer capacity, should be within 1 pH unit of the buffer. The acetic acid/acetate pair are most appropriate for a buffer with pH 5.00. [CH₃COONa] [CH₃COONa] log = = 1.800 = 1.8 [CH₃COOH] [CH₃COOH] Since we are making a total of 1L of buffer, let y = vol CH₃COONa and (1-y) = vol 1.800 = [CH₃COONa] = = 0.10 y [CH₃COOH] [0.10 0.10-0.10 y 1.800(0.10 - 0.10 y) = 0.10 y; 0.1800 = 0.2800 y; y = 0.6429 = 0.64 L 640 mL of 0.10 M CH₃COONa, 360 mL Check. pH (buffer) > (CA) and the calculated amount of CB in the buffer is greater than the amount of CA. Acid-Base Titrations (section 17.3) 17.33 (a) Curve B. The initial pH is lower and the equivalence point region is steeper. (b) pH at the approximate equivalence point of curve A = 8.0 pH at the approximate equivalence point of curve B = 7.0 (c) Volume of base required to reach the equivalence point depends only on moles of acid present; it is independent of acid strength. Since acid B requires 40 mL and acid A requires only 30 mL, more moles of acid B are being titrated. For equal volumes of A and B, the concentration of acid B is greater. 17.34 (a) The quantity of base required to reach the equivalence point is the same in the two titrations, assuming both sample solutions contain equal moles of acid. 514