ph = pk a + log 10{[base]/[acid]}

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 ln(p) = - H vap + C ln(p 2/p 1) = - ( H vap/r) { (1/T 2) - (1/T 1) } T 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 H = U + pv G = H - TS G rxn = G rxn + RT ln Q ln K = - G rxn/rt K p = K C (RT) n 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 EXAM Name Panthersoft ID Signature Part 1 (24 points) Part 2 (52 points) Part 3 (44 points) TOTAL (120 points) 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) Which of the following hydroxide compounds is a strong soluble base? a) KOH (potassium hydroxide) b) AgOH (silver hydroxide) c) Ni(OH) 2 (nickel II hydroxide) d) Both a and b e) Both a and b and c 2) From the substances HI, H 2S, and H 2Se a) HI is the strongest acid and H 2Se is the weakest acid b) HI is the strongest acid and H 2S is the weakest acid c) H 2S is the strongest acid and HI is the weakest acid d) H 2S is the strongest acid and H 2Se is the weakest acid e) H 2Se is the strongest acid and H 2S is the weakest acid 3) In the reaction Al(H 2O) 6 3+ (aq) + H 2O( ) Al(H 2O) 5OH 2+ (aq) + H 3O + (aq) the Al(H 2O) 6 3+ ion functions as a) the conjugate base of H 2O b) the conjugate acid of H 2O c) a Bronsted base d) a Bronsted acid e) none of the above 4) A titration curve is given in the figure below. The titration is carried out at T = 25. C. Based on this curve, we may say that a) a strong acid is being titrated with a strong base b) a weak acid is being titrated with a strong base c) a strong base is being titrated with a strong acid d) a weak base is being titrated with a strong acid e) cannot tell from the information given 3

4 5) A small amount of a strong acid is added to a buffer solution. What will happen to the ph of the solution? a) The ph will increase by a large amount b) The ph will decrease by a large amount c) The ph will increase by a small amount d) The ph will decrease by a small amount e) The ph will not change 6) Calcium carbonate (CaCO 3) is a slightly soluble ionic compound. For which of the following solutions will the number of grams per liter of CaCO 3 that will dissolve be largest (at T = 25. C)? a) A M solution of hydrochloric acid (HCl) b) A M solution of sodium chloride (NaCl) c) A M solution of sodium hydroxide (NaOH) d) A M solution of calcium nitrate (Ca(NO 3) 2) e) pure water Part 2. Short answer. 1) Values for K b for several weak bases are given below, at T = 25. C. NH 3 K b = 1.8 x 10-5 (C 2H 5) 2NH K b = 5.4 x 10-4 C 5H 5N K b = 1.7 x 10-9 C 6H 5NH 2 K b = 3.8 x From the lists below circle the correct answer. There is one and only one correct answer per problem. [4 points each] a) The strongest weak base NH 3 (C 2H 5) 2NH C 5H 5N C 6H 5NH 2 b) The strongest conjugate acid NH 4 + (C 2H 5) 2NH 2 + C 5H 5NH + C 6H 5NH 3 + 2) An aqueous solution of a weak acid has ph = 5.18 at T = 25. C. What are the values for poh and [H 3O + ] for the solution? [3 points each] poh = [H 3O + ] = 3) Rubidium hydroxide (RbOH) is a strong soluble base. Find the ph of a M aqueous solution of rubidium hydroxide at T = 25. C. [8 points] 4

5 4) A buffer is formed by adding an equal number of moles of hypochlorous acid (HClO, K a = 3.5 x 10-8 ) and potassium hypochlorite (KClO) to water, at T = 25. C. a) What is the ph of the buffer? [5 points] b) Give the balanced reaction that takes place when a small amount of potassium hydroxide (KOH) is added to the above buffer. [5 points] 5) Give the oxidation number for nitrogen (N) in each of the following molecules. [4 points each] HNO 3 NF 3 6) Balance the following oxidation-reduction reactions for acid conditions. [12 points each] 5

6 Part 3. Problems. 1) Hypochlorous acid (HOCl) is a weak acid, with K a = 3.5 x 10-8 at T = 25. C. a) Give the conjugate base of HOCl (correct formula and charge) [4 points] b) What is the ph and the percent dissociation for a M aqueous solution of HOCl at T = 25. C? [12 points] 6

7 2) A g sample of a weak monoprotic acid is titrated with a M solution of NaOH, a strong base. After ml of the NaOH solution has been added the equivalence point for the titration is reached. What is the molecular mass of the weak monoprotic acid? [14 points] Let HA = weak acid HA(aq) + NaOH(aq) NaA(aq) + H 2O( ) 7

8 3) A chemist prepares L of a M solution of iodoacetic acid (CH 2ICOOH, MW = g/mol ), a weak monoprotic acid with K a = 7.6 x 10-4 at T = 25. C. How many grams of sodium iodoacetate (NaCH 2ICOO, MW = g/mol) must be added to the solution to convert it into a ph = 3.00 buffer? [14 points] 8