MSJ2e_Ch17_ISM

MSJ2e_Ch17_ISM - Chapter 17: Additional Aqueous Equilibria...

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Chapter 17: Additional Aqueous Equilibria 709 Chapter 17: Additional Aqueous Equilibria Questions for Review and Thought Review Questions 1. The buffer capacity of a buffer solution is related to the concentrations of the buffer’s acid and base. The buffer solution is able to resist changes in pH only until the total amount of added acid or added base completely neutralizes one component of the buffer solution. The higher the buffer concentrations are, the higher the buffer capacity is. 2. A weak acid-base conjugate pair must be in the solution. The weak base can neutralize added acid making more of the weak base, and the weak acid can neutralize added base making more of the weak acid; hence, the pair together makes a buffer solution capable of resisting pH. (a) A strong acid, HCl, and a weak acid, CH 3 COOH, together in a system are able to neutralize added base, but are not able to neutralize added acid, so this pair does not form a buffer. (b) The dissolved NaH 2 PO 4 salt contains the weak acid H 2 PO 4 , and the dissolved Na 2 HPO 4 salt contains its conjugate weak base, HPO 4 2– , so this pair would form a buffer. (c) The solution contains the weak acid H 2 CO 3 and a dissolved salt, NaHCO 3 , containing its conjugate weak base, HCO 3 , so this pair would form a buffer. 3. Analyze these pairs as was done in the answer to Question 2. (a) A strong base (NaOH) and a neutral salt (NaCl) together in a system are able to neutralize added acid, but are not able to neutralize added base, so this pair does not form a buffer. (b) A strong base (NaOH) and a weak base (NH 3 ) together in a system are able to neutralize added acid, but are not able to neutralize added base, so this pair does not form a buffer. (c) The dissolved Na 2 HPO 4 salt contains the weak acid HPO 4 2– , and the dissolved Na 3 PO 4 salt contains its conjugate weak base, PO 4 3– , so this pair would form a buffer. 4. For a solution to be a buffer, a weak acid-base conjugate pair must be in the solution. The weak base can neutralize added acid making more of the weak acid, and the weak acid can neutralize added base making more of the weak base; hence, the pair together makes a buffer solution capable of resisting pH. For example, a buffer can be composed of the weak base, NH 3 , and the soluble salt NH 4 Cl, which provides the conjugate acid, NH 4 + , together in a solution. When a strong acid, HCl, is added, the weak base, NH 3 , is able to neutralize it: NH 3 + HCl(aq) NH 4 Cl(aq) When a strong base, NaOH, is added, the weak acid, NH 4 Cl, is able to neutralize it: NH 4 Cl(aq) + NaOH(aq) NH 3 + NaCl(aq) 5. The “end point” is the term used during an acid base titration describing the volume of added titrant at which an appropriate indicator being used changes color. The “equivalence point” is the point during a titration describing the volume of added titrant at which the moles of reactant acid are equal to the moles of reactant base. These two points ought to be very close to the same, if the indicator is selected properly, but in practice they will not to be exactly the same.
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MSJ2e_Ch17_ISM - Chapter 17: Additional Aqueous Equilibria...

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