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Homework 7-solutions - hernandez(ah29758 Homework 7...

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hernandez (ah29758) – Homework 7 – sutcliffe – (51045) 1 This print-out should have 18 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. All about buffers! 001 10.0points Some solid KClO 2 is added to a water solution of HClO 2 . Which of the following statements is correct? 1. [OH - ] of the solution decreases. 2. pH of the solution decreases. 3. pH of the solution increases. correct 4. [H 3 O + ] will remain the same. Explanation: 002 10.0points Which response identifies the statements true of buffer solutions? l) A buffer solution could consist of equal concentrations of ammonia and ammo- nium bromide. 2) A buffer solution could consist of equal concentrations of perchloric acid (HClO 4 ) and sodium perchlorate. 3) A buffer solution will change only slightly in pH upon addition of small amounts of acid or base. 4) In a buffer solution containing benzoic acid (C 6 H 5 COOH) and sodium benzoate (NaC 6 H 5 COO) the species that reacts with added hydroxide ion is the benzoate ion. 1. 2, 3, 4 2. 1, 4 3. 2, 3 4. Another combination 5. 1, 3 correct Explanation: HA + H 2 O A - + H 3 O + B + H 2 O BH + + OH - NH + 4 is the conjugate acid of the weak base NH 3 ; C 6 H 5 COO - is the conjugate base of the weak acid C 6 H 5 COOH. HClO 4 is a strong acid. OH - ions introduced into an acid buffer system removes the H 3 O + . Buffers are composed of comparative amounts of a weak acid/base and its conjugate and allows only a small change in pH when an acid or base is added. 003 10.0points Which of the following mixtures can produce a buffer? I) NH 3 + NaOH II) NH 3 + NH 4 Cl III) H + + OH - IV) HClO 2 + NaClO 2 V) (CH 3 ) 3 NHCl + (CH 3 ) 3 N 1. II, IV, and V only correct 2. IV only 3. II, III, and IV only 4. I, III, and V only 5. II only Explanation: A buffer contains a weak acid or weak base, plus the salt of that weak acid or base; or, a mixture which will have this composition after any acid-base reactions occur. 004 10.0points What is the pH of a solution in which the concentration of hypochlorous acid is 0.50 M and the concentration of sodium hypochlorite is 1.0 M? ( K a for HOCl = 3 . 2 × 10 - 8 .) 1. 7.19 2. 7.49 3. 5.44
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hernandez (ah29758) – Homework 7 – sutcliffe – (51045) 2 4. 8.31 5. 7.80 correct 6. 6.89 7. 8.10 Explanation: [OCl - ] = 1.0 M [HOCl] = 0.50 M K a = 3 . 2 × 10 - 8 pH = p K a + log parenleftbigg [OCl - ] [HOCl] parenrightbigg = - log(3 . 2 × 10 - 8 ) + log parenleftbigg 1 . 0 0 . 5 parenrightbigg = 7 . 79588 005 10.0points A buffer (pH 3.70) was prepared by mixing 1.00 mole of formic acid and 1.00 mole of sodium formate to form an aqueous solution with a total volume of 1.00 L. To 400 mL of this solution was added 50.0 mL of 1.00 M NaOH. What is the pH of this solution?
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