FORMULA SHEET (tear off)

Transcription

1 FORMULA SHEET (tear off) N A = x C = ( 5 / 9) ( F - 32) F = ( 9 / 5)( C) amu = x kg C = K K = C atm = 760 torr = 760 mm Hg 1 atm = bar pv = nrt R = L atm/mol K 1 L atm = J R = J/mol K 1 J= 1 kg m 2 /s 2 p A = X A p A [B] = k p B p A = X Bp A T b = K b m B T f = K f m B = [B]RT K p = K C (RT) n H = E + pv G rxn = G rxn + RT ln Q G = H - TS ln K = - G rxn/rt If ax 2 + bx + c = 0, then x = ( - b [b 2-4ac] 1/2 ) 2a K a. K b = K w = 1.0 x (at T = 25 C) ph = pk a + log 10{[base]/[acid]}

2 GENERAL CHEMISTRY 2 SECOND HOUR EXAM (Sample) Name Panthersoft ID Signature Part 1 (20 points) Part 2 (50 points) Part 3 (50 points) TOTAL (120 points) j Do all of the following problems. Show your work.. 2

3 Part 1. Multiple choice. Circle the letter corresponding to the correct answer. There is one and only one correct answer per problem. [4 points each] 1) A Bronsted base is a) a proton acceptor b) a proton donor A c) an electron pair acceptor d) an electron pair donor e) any ionic compound that will dissolve in water 2) Ammonium perchlorate (NH 4ClO 4) is a soluble salt formed by the reaction of a strong acid with a weak base. The ph of a M solution of ammonium perchlorate, measured at T = 25. C, is expected to be a) exactly equal to 7.0 b) approximately equal to 7.0 D c) significantly larger than 7.0 d) significantly smaller than 7.0 e) undefined, because the concept of ph does not apply to solutions of salts 3) Hypoiodous acid (HOI) is a weak acid, with K a = 3.5 x 10-8 (at T = 25. C). OI - ion is a) a weak acid b) a strong acid C c) a weak base d) a strong base e) None of the above, as ions have no acid or base properties 4) Which of the following is the strongest oxyacid? a) HIO 2 b) HIO 3 E c) HBrO d) HBrO 2 e) HBrO 3 5) Consider the following three aqueous solutions Solution A Solution B Solution C moles of HClO 2 (chlorous acid) moles of NaClO 2 (sodium chlorite) dissolved in water, with final volume V = ml moles of HClO 2 (chlorous acid) moles of NaOH (sodium hydroxide) dissolved in water, with final volume V = ml moles of HClO 2 (chlorous acid) moles of NaCl (sodium chloride) dissolved in water, with final volume V = ml Which of the sbove solutions is a buffer solution? a) Solution A only b) Solution B only D c) Solution C only d) Both solution A and solution B e) Both solution A and solution C 3

4 Part 2. Short answer. 1) The ph of an aqueous solution is ph = 8.82 at T = 25 C. Find [H 3O + ], [OH - ], and the poh for the solution. [8 points] poh = ph = 5.18 [H 3O + ] = 10 -ph = = 1.5 x 10-9 M [OH - ] = 10 -poh = = 6.6 x 10-6 M 2) A chemist prepares an aqueous solution of nitrous acid (HNO 2), a weak acid. To determine the concentration of nitrous acid in the solution, she titrates a ml sample of the solution with a M solution of sodium hydroxide (NaOH), a strong soluble base. The titration was carried out at T = 25. C. The equivalence point for the titration occurs when the volume of added sodium hydroxide solution is ml. a) What is the concentration of nitrous acid in the weak acid solution prepared by the chemist? [6 points] Reaction is HNO 2 + NaOH NaNO 2 + H 2O Based on this, at the equivalence point moles HNO 2 = moles NaOH (M acid ) (20.00 x 10-3 L) = ( M) (23.81 x 10-3 L) M acid = (23.81/20.00) ( M) = M b) Which of the following indicators would be the best choice to use in the above titration (circle the correct answer). [3 points] bromophenol blue methyl red bromothymol blue phenolphthalein pk a = 3.8 pk a = 5.2 pk a = 6.6 pka = 9.0 4

5 3) Answer each of the following questions by filling in the blank. [4 points each] a) The conjugate base of HCO 3-. CO 3 2- b) The value for K b for NO 2 -., at T = 25. C 2.2 x (Note K a = 4.5 x 10-4 for HNO 2 at T = 25. C) c) The ph of pure water at T = 50. C (Note K w = 5.5 x at T = 50. C) 4) For each of the following questions circle the correct answer. There is one and only one correct answer per question. [3 points each] The weakest acid HI HBr HCl HF A strong soluble base AgOH Cu(OH) 2 Ba(OH)2 Fe(OH) 3 An example of a polyprotic acid HBr HF HNO 2 H2SO4 5) What is the oxidation number for sulfur (S) in each of the following molecules or ions? [3 points each] H 2S -2 S 2O H 2SO 3 +4 S 8 0 5

6 Part 3. Problems. 1) A chemist prepares ml of a M aqueous solution of hypochlorous acid (HOCl, K a = 3.5 x 10-8 ). a) What is the ph of the above solution? [10 points] Reaction is HOCl + H 2O H 3O + + OCl - K a = [H 3O + ][OCl - ] = 3.5 x 10-8 Initial Change Equil. [HOCl] HOCl x x H 3O + 0 x x OCl - 0 x x So (x)(x) = 3.5 x 10-8 Assume x << , then ( x) x 2 = 3.5 x 10-8 x 2 = (0.0200)(3.5 x 10-8 ) = 7.0 x x = [H 3O + ] = (7.0 x ) 1/2 = 2.65 x 10-5 M (Note that x is small is a correct assumption) ph = - log 10(2.65 x 10-5 ) = 4.58 b) moles of potassium hydroxide (KOH, a strong soluble base) is added to the above solution of hydrochlorous acid. After the addition of KOH, what is the new value for ph? You may assume that the volume of the solution remains constant at V = ml. [8 points] We have the neutralization reaction, which goes to completion (because the base is a strong base) HOCl + KOH K + + OCl - + H 2O initial moles HOCl = ( M) ( L) = mol initial moles KOH = mol After neutralization moles NaOH = 0 (limiting reactant) moles HOCl = ( ) = mol moles OCl - = mol [HOCl] = M [OCl - ] = M We have significant amounts of weak acid and conjugate base, and so can use the Henderson equation (we can also do this problem using our usual ICE table) ph = pk a + log 10{[base]/[acid]} = - log 10(3.5 x 10-8 ) + log 10{( M)/( M)} = ( ) =

7 2) The following question concerns the slightly soluble ionic compound silver iodide (AgI, K sp = 8.3 x ). a) A student adds excess solid silver iodide to water, and shakes the resulting mixture until no more silver iodide dissolves. What is the concentration of Ag + ion present in the solution? [8 points] The solubility reaction is AgI(s) Ag + (aq) + I - (aq) K sp = [Ag + ] [I - ] = 8.3 x Initial Change Equil Ag + 0 x x I - 0 x x (x)(x) = x 2 = 8.3 x x = [Ag + ] = (8.3 x 10-17) 1/2 = 9.1 x 10-9 M b) A student adds excess solid silver iodide to a M solution of sodium iodide (NaI), and shakes the resulting mixture until no more silver iodide dissolves. What is the concentration of Ag + present in the solution? [8 points] This problem is the same as in part a, except we now have an initial concentration of I - from the dissolution of NaI (a strong electrolyte) NaI(s) Na + (aq) + I - (aq) [I - ] = M Initial Change Equil Ag + 0 x x I x x (x)( x) = 8.3 x Assume x << x (0.0400) = 8.3 x x = [Ag + ] = 8.3 x = 2.1 x M (note x is small is a correct assumption) 7

8 3) Balance the following unbalanced oxidation-reduction reactions [8 points each] a) SO 2-3 (aq) + MnO - 4 (aq) SO 2-4 (aq) + Mn 2+ (aq) (for acid conditions) ox SO 2-3 (aq) + H 2O( ) SO 2-4 (aq) + 2 H + (aq) + 2 e - x 5 red MnO 4- (aq) + 8 H + (aq) + 5 e - Mn 2+ (aq) + 4 H 2O( ) x 2 net 5 SO 3 2- (aq) + 2 MnO 4 - (aq) + 6 H + (aq) 5 SO 4 2- (aq) + 2 Mn 2+ (aq) + 3 H 2O( ) b) Al(s) + NO 2 - (aq) AlO 2 - (aq) + NH 3(aq) (for base conditions) ox Al(s) + 2 H 2O( ) AlO 2 - (aq) + 4 H + (aq) + 3 e - x 2 net (acid) red convert to base NO 2 - (aq) + 7 H + (aq) + 6 e - NH 3(aq) + 2 H 2O( ) 2 Al(s) + NO 2 - (aq) + 2 H 2O( ) 2 AlO 2 - (aq) + H + (aq) + NH 3(aq) OH - (aq) OH - (aq) net base 2 Al(s) + NO 2 - (aq) + H 2O( ) + OH - (aq) 2 AlO 2 - (aq) + NH 3(aq) 8