Chemistry 265 December Exam 2011 Smith-Palmer

Transcription

1 1 Chemistry 265 December Exam 2011 Smith-Palmer NAME: [1] 1. Define an anode [1] Define a cathode [2] What is the E o for the following reaction: Ag + + Cu Ag (s) + Cu 2+ Ag + + e - Ag (s) E o = V Cu e - Cu(s) E o = V [2] What is the equilibrium constant for the reaction?

2 2 [5] 2. Calculate the potential of the following cell and indicate whether it is galvanic or electrolytic. Pt V 3+ (8.00 x 10-4 M), VO 2+ ( M), H + ( M) V(OH)4 + (0.100 M), VO 2+ (1.00 x 10-3 M), H + ( M) Pt VO H + + e - V 3+ + H2O V(OH) H + + e - VO H2O E o = V E o = 1.00 V

3 3 3. No more than two of the following three constituents can exist in appreciable amount in any solution: sodium carbonate, sodium bicarbonate and sodium hydroxide. [2] What happens if sodium hydroxide is mixed with sodium bicarbonate? Write the equation. You can work out the composition of the solution by titrating with HCl and using the following two indicators: phenolphthalein which changes colour in an alkaline range, and bromocresol green which changes in an acid range. With one particular solution containing some mix of sodium carbonate, sodium bicarbonate and sodium hydroxide, titration of a 50.0 ml portion to a phenolphthalein endpoint required 22.1 ml of M HCl. [2] If the mixture only contained sodium hydroxide, how much M HCl would be required if the titration was repeated using bromocresol green as the indicator? [2] If the mixture contained only sodium carbonate, how much M HCl would be required if the titration was repeated using bromocresol green as the indicator? [3] Actually, the titration to a bromocresol green endpoint required 48.4 ml. Use the results of the two titrations (50.0 ml portion required 22.1 ml of M HCl to reach a phenolphthalein endpoint, another 50.0 ml portion required 48.4 ml of M HCl to reach a bromocresol green endpoint) to calculate the concentrations of the two constituents.

4 4 [12 ] 4. WRITE NOTES EXPLAINING OR DEFINING 4 OF THE FOLLOWING (i) Variance (ii) buffer capacity (iii) Beers law (iv) calomel electrode (v) primary standard

5 5 (vi) activity coefficient (vii) water hardness (ix) ph electrode

6 6 [15 ] 5. EXPLAIN 5 OF THE FOLLOWING USE NOTE FORM ANSWERS. (i) Why did we add stannous chloride to our solution of iron before we titrated the iron with ceric sulphate? And why immediately before we did the titration? (ii) What is the double advantage of using an NH 3 buffer in the EDTA titration of ions such as Ni 2+ or Zn 2+? (iii) How does a masking agent work? When is one required? Give an example of one and its application.

7 7 (iv) Describe how a titration of Cu 2+ using EDTA could be followed using a potentiometric method. (v) What is meant by an auxiliary oxidizing agent or an auxiliary reducing agent? (vi) Draw a block diagram for a spectrophotometer. Write an equation which shows the relationship between A, P and P o.

8 8 (vii) How did we separate cations on a cation exchange resin? [3 ] 6. ANSWER 3 OF THE FOLLOWING QUESTIONS: (a) When is perchloric acid dangerous? (b) Why is it important to grind samples before sampling? (c) What is the formula used to calculate the standard deviation in a sampling operation when there are two types of particles involved?

9 9 (d) What silver salt is soluble? [1] 7. What is the concentration of nitrogen in the air? [1] What is the concentration of carbon dioxide in the air? [2] Write the equations for the dissolution of carbon dioxide in water. What is the approximate resulting ph?

10 10 [5] 8. What volume (how many mls) of M NaOH should be added to 25.0 ml of M salicylic acid to adjust the ph to 3.50? Ka for salicylic acid = 2.80

11 11 [2] 8. Name two heavy metals that cause problems in the environment. [2] A complexing agent is sometimes used to treat heavy metal poisoning. Give an example of a complexing reagent. What is the stoichiometry of its reaction with metal ions? [2] Put the following ions in the order of their activity coefficients (high to low) if they are all in the same solution. Al 3+, Ca 2+, Ce 4+, K +, Li +, Na + Will the activity coefficients increase, decrease or stay the same when the ionic strength is increased

12 12 [25] 9. DO 5 OF THE FOLLOWING PROBLEMS: (a) A technician obtained the following results for the concentration of cholesterol in a blood sample: 240, 265, 230, 238 and 244. Can any result be rejected by the Grubbs test? Show your calculations, explain your decision. Table: Critical values of G for rejection of outlier Number of G (95 % observations confidence interval (b) The ph of a 0.10 M solution of the salt NaX (HX is a weak acid) is Calculate the dissociation constant of HX.

13 13 (c) Calculate the solubility of BaSO 4 in 0.02 M K 2 SO 4. K sp for BaSO 4 = (d) Cocaine is a weak base, K b = Representing the base as B and the hydrochloride salt as BHCl, calculate the ph of a solution made by mixing 60 ml of M BHCl with 40 ml of M NaOH.

14 14 (e) Calculate the value of p Ni 2+ at the equivalence pt for the titration of mole of Ni 2+ with EDTA of ph 4. Total volume is 100 ml. At a ph of 4, α 4 for EDTA = K NiY = (f) ml of KIO 3 reacted with exactly ml of M KI in acid medium according to the following unbalanced equation: Balance the equation and calculate the molarity of the KIO 3 solution.

15 15 (h) The organic compound 2,3-dimethoxybenzaldehyde (MW 166.2) exhibits an ultraviolet absorption band at 220 nm in ethanol solution. A solution containing mg of the compound in 20.0 ml ethanol gave an absorbance of 1.73 in a 2.00cm cell. Calculate the molar absorptivity of the compound at 220 nm.

16 16 [10] 10. DO TWO OF THE FOLLOWING PROBLEMS (a) An acidic solution of KMnO 4 is standardized with primary sodium oxalate. If a mg (2.100 mmol) sample of Na 2 C 2 O 4 requires ml of KMnO 4 solution to reach the equivalence point determine the molarity of the KMnO 4 solution. The unbalanced equation for the reaction is: KMnO 4 + Na 2 C 2 O 4 Mn 2+ + CO2

17 17 (b) There are two solubility calculations for this problem. Calculate the molar solubility of Al(OH) 3 K sp = (Note: K sp is small, contribution of OH from dissolution to overall [OH ] will be negligible.) Also calculate the molar solubility of Mg(OH) 2 K sp = (Note: K sp is moderate, the overall contribution of the water to the overall [OH ] will be negligible)

18 18 (c) The mineral cinnabar consists of red HgS (K sp = ). When it dissolves in H 2 O, a variety of equilibria are set up. Write equations for all the equilibria you might expect to be present in such a solution, each with the associated equilibrium constant. For H 2 S, K a1 = and K a2 = Write the charge balance equation for the solution. Write the mass balance for the mercury ions in terms of sulfides. The ph is fixed (buffered) at Write an expression for [HS - ] in terms of [S 2- ] Write an expression for [H 2 S] in terms of [S 2- ]