Chemistry

Typical Exams By H. M. FAHMY

DEPARTMENT OF CHEMISTRY , FACULTY OF SCIENCE ,

CAIRO UNIVERSITY

PRE-PHARMACY STUDENTS MID TERM EXAM

[GROUP 4 PROF. H. M. FAHMY] DECEMBER 1994

PHYSICAL CHEMISTRY

Time: 45 minutes

Answer the following questions .

1-Match RHS with LHS {4 marks}

2-What is the density expressed in g/l of CO₂ gas at 100 ^oC and 0.5 atm.{3 marks}

3-Which gas will diffuse more rapidly CO₂ or NO₂ comment.{2 marks}

4-Ethanol(B.p. 78.5 ^oC) and water form a constant B.p. mixture having a B.p. of 78.2 ^oC and a composition of 95.6% ethanol. Draw the vapour pressure composition and the boiling point composition diagrams of this system and give your comments. {4 marks}

5-What of the following observations illustrates colligative properties {2marks}

a-Pure benzene boils lower than water by 20 ^oC

b-The taste of 0,1 M solution of NaCl is different from that of 0.1 M glucose solution at room temperature

c-Saturated water with salt boils at higher temperature than pure water at 760 mm Hg

d-Pure benzoic acid freezes at 122 ^oC while pure water freezes at 0 ^oC

e-The osmotic pressure of 1.0 M NaCl is nearly twice that of 1.0 M glucose 

6-Define the following by a sentence or a mathematical expression {5 marks}

a-Molal Concentration   b- Moles   c-Non ideal solution   d-The Van’t Hoff’s factor

e-Henry’s law  f-Raoult’s law  g-Osmotic pressure  h-Elevation in boiling point

i-Distribution coefficient  j-Surface tension.

Total 20 marks

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DEPARTMENT OF CHEMISTRY , FACULTY OF SCIENCE ,

CAIRO UNIVERSITY

PRE-PHARMACY STUDENTS MID TERM EXAM

[GROUP 4 PROF. H. M. FAHMY] APRIL 1995

PHYSICAL CHEMISTRY

Time: 45 minutes

Answer four ONLY of the following questions

 I)State wether its right or wrong                                                                        (5 marks)

1-NH₄Cl suppreses the ionization of NH₄OH

2-S for Al(OH)₃ equals (K_s / 4)^1/3

3-If DE is -ve the reaction is exothermic

4-The degree of ionization a is given by l / l_∞

5- The units of k are Ω ^-1 / cm

6- The amount of heat evolved of a neutralization reaction is always equal to -13700 cal

7-For the reaction H₂O(l) = H₂O (v) at 100 ^o C Dn = -1

8-In the reaction 2CO(g) + CO(g) = 2 CO₂(g) DH, DE and Dn are -ve

9-In a buffer solution pH = pK_a  

10- Ka & Kb are ≈ a ² C and  a equals l / l_∞

11- The strengh of a 0.1M HCl solution = 3.65 g/l

12- Precipitation occures when S of a sparingly soluble salt exceeds its K_s

13- The pH of 0.1M HCl is 1

14-The mass of NaOH (M=40) required to prepare a 250 ml of NaOH of pH 13 is 1g

15-The resistance of a conducting solution is given by Kohlrauch’s law

II) Will a ppt be formed when a AgNO₃ solution of [Ag⁺]=10^-2 M is added to a 0.02M of CH₃COONa  knowing that K_s of CH₃COONa is 2X10^-3 .Give your answer in the form of YES or NO and WHY                                                                                                                  (5 marks)

III) To 35 ml of 0.02M acetic acid were added 10 ml of 0.05M NaOH.  What is the pH of the solution (pK_a of acetic acid = 4.74) .Give your answer in the form: pH =number + log number.                                                                                                                                (5 marks)

IV)The specific conductance of 0.02N propionic acid in water iat 25 ^o C is 0.00044 Ω ^-1 cm^-1 .  The equivalent conductance at infinite dilution is 440 Ω ^-1 cm² g eq^-1.  Calculate the acid dissociation constant and pH of solution .                                                                                               (5 marks)

V) The standard heat of combustion of liquid ethanol is -326.7 Kcal at 25 ^o C .  Calculate the heat of formation .  Given that the heat of formation of CO₂ is -94.05 Kcal and that of H₂O is -68.32 Kcal .                                                                                                                           (5 marks)

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CAIRO UNIVERSITY FACULTY OF SCIENCE

CHEMISTRY DEPARTMENT

PRE-PHARMACY EXAMINATION

PHYSICAL & INORGANIC CHEMISTRY

                                                             Time allowed: 2 hours

Answer Five Only of the Following Questions

1)-a-Knowing that the heat of combustion of C₆H₆ equals to -782 K.cal at 25 ^oC , calculate the change in internal energy of the reaction.

b-The specific conductivity of a saturated solution of AgCl at 20^oC is 1.33 X 10^-6 Ω^-1 cm^-1. If the ionic conductivity of Ag⁺ and Cl^- are 56.6 and 68.4 Ω^-1 cm² eq^-1 respectively .  Calculate   i) Solubility  

 and   ii) Solubility product.

c-Calculate the pH of a 0.2M KCN solution.  If K_a of HCN is 4.9 X 10^-10 what is the degree of hydrolysis

of the salt.

d-Write brief points on one of the following

i) Heat of formation     ii) Acid - base indicators

2)-a-A certain water solution has a pH of 6.8 at 60 ^oC.  Is this water solution acidic, basic or neutral knowing that K_w at 60 ^oC equals to 10^-13. Explain

b-Write breif points on one of the following

i)Water is not considered as a buffer solution

ii)The common ion effect in the precipitation of a mixture containing group II cations

c-Calculate K_b for NH₃(aq) if [OH^-] equals to 1.9 X 10^-3 in a 0.2M solution.

d- The equivalent conductance of a solution of 1.02 X 10^-3 g eq l^-1 of acetic acid is 48.15 Ω^-1 cm²at 25 ^oC and L_∞  equals 390.7 Ω^-1 cm².  Calculate the degree of dissociation of the acid

3)-Choose the correct answer and write it in your answering book

i-Which of the following combinations would give a buffer solution

a-HNO₃ + NaNO₃        b-NaOH + NaNO₃       c-NH₄OH +NH₄NO₃       d-None of a,b,c

ii-Lemon juice has a pH of 2.3 the [H+] equals to

a-10^-2.3            b-10 ^2.3            c-2.3 X 10        d-None of a,b.c

iii-The expression for Ksp for As₂S₃ is

a-[As₂ ³⁺ ] [S₃^2- ]          b-[As³⁺]² [S^2-]³             c-[As³⁺]³ [S^2-]²             c-None of a,b,c

iv-CaSO₄ has a K_sp of 3 X 10^-5. In which of the following CaSO₄ is most soluble

a-1M K₂SO₄                 b-2M CaCl₂                 c-Pure H₂O                 d-Same in all

v-K_b for NH₃ is 2 X 10^-5. K_a for NH⁴⁺ ion is

a-2 X 10^-5        b-5 X 10^-a        c-5 X 10^-10       d-2 X 10^-19

vi-In which one of the following systems is the amount of stored energy is the largest under 1atm,

a-1mol H₂O(l) at 0 ^oC b-1mol of H₂O(l) at 100 ^oC      c-1mol H₂O (g )at 100 ^oC      d-None of a,b,c

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 M. Sc. EXAMS

MULTIPLE CHOICE EXAMS

Choose the correct answer alphabetic letter and write it in your answer book.

1)         If a dimer is separated in acid medium in good yield with the main product of electrolysis how many pathways can be suggested.

            a- 2 pathways              b- 3 pathways               c- 4 pathways

2)         Residual currents in polarography are known as.

            a- Non Faradaic currents          b- Faradaic currents     c- Impurity current

3)         The observed potential in RCOO⁺ ® RCOO^. + e^- is 2.46 vs. HE while using TBA as mediator the potential will be .

             a- Higher         b- Lower          c- None of a & b

4)         A multiple electroactive sites organic compound is to be reduced in only one sight. The proper electrolysis technique should be

            a- CPE             b- CCE

5)         dE1/2 /dpH equals 0.0591 for

            a- 1e^- & 1H⁺    b- 2e^- & 2H⁺    c- None of a & b

6)         Convection currents are due to

            a- Concentration gradients        b- Electric field c- Disturbances

7)         The decrease in current with increase of time is

            a- Linear          b- Sinusoidal     c- Exponential

8)         The Cottrel equation is simply expressed by

            a- il µ  C         b- id µ C         c- i  µ  C/Öt     d- None of a,b & c

9)         The DME , over the limiting current plateau of a polarographic wave, behaves as

            a- A depolarized electrode        b- A completely polarized electrode     

10)       One equivalent of a substance is deposited by the passage of

            a- 26.8 Ah       b-1Ah              c-1coulomb                  d- 96500/n

11)       C₆H₅CH₃ can be converted to C₆H₅CHO by CeIV / CeIII system in

            a- Alkaline media          b- pH 7            c- Highly acidic medium

12)       Is the following sequence of equations in a  mediated electrochemical oxidation of benzene is missing a step

            Fe³⁺  +  e^-  =  Fe²⁺

            Fe²⁺  + H₂O₂  =  OH^-  +  OH^.  +  Fe²⁺

            OH^.  +  C₆H₆  =  [ C₆H₅OH ]^.

            [C₆H₅OH ]^.  + Fe³⁺  =  C₆H₅0H  +  H⁺  +  Fe²⁺

                a- Yes              b- No

13)       The DME, over the limiting current plateau , behaves as

            a- A depolarized electrode        b- A polarized electrode           c- None of a & b

14)       Hg is used as a perfect electrode for electrooxidation processes since it undergoes anodic dissolution at + 0.25 V vs. SCE

            a- Yes              b- No

15)       In a CPE experiment the decrease in current with increase of time is

            a- Linear          b- Exponential              c- Reversible

16)       A square diagram for a 2 electron and 2 proton transfer has

            a- 6 pathways & 7 intermediates           b- 6 pathways & 6 intermediates

            c- 7 pathways & 7 intermediates           d- 7 pathways & 6 intermediates

17)       The Cottrell equation simply means that

            a- i ∞ C                        b- i ∞ Ct                      c- i ∞ C√t         d- None of a, b, c

18)       Convection current is due to

            a- Concentration gradient                      c- Electric field              c- None of a,b

19)       i_d = nFADC_b  / d

            a- Yes              b- No

20)       The Ni electrode is considered to be a powerful anode and the mechanism of indirect oxidation is through

            a- Direct reaction with Ni          b- Formation of Ni(OH)₂          c- Formation of NiOOH                        c- Attack with NiO₂

21)       A modified electrode can used and considered as an advanced type of external mediator

            a- Yes              b- No

22)       A mediator should be electrochemically active and with a reaction k⁰ > 10^-1 cm s^-1

            a- Yes              b- No

23)       W = QM / anF where  a is the transfer coefficient with a value 1>a>0

            a- Wrong         b- Right

24)       The Randles-Cevcik equation is i_d = 708 n √D t^1/6 m^2/3 C_b

            a- Yes              b- No

25)       Epc/2 - Epa/2 =0.056 / n for a reversible reaction

            a- Yes              b- No

26)       A pronounced difference exists between E1/2 measured by a microelectrode and a large surface electrode for  a reversible system

            a- Yes              b-No

27)       The reaction between a species and a mediator is of homogenuous nature

            a- Wrong         b- Right

28)       i - E curves from stationary electrodes are peaked while those from rotating electrodes are smooth

            a-Yes               b- No

29)       The rate of diffusion according to Fick’s first law of diffusion is p[roportional to

 dC / dx

            a- Wrong         b- Right

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PROBLEMS FOR M.S. STUDENTS 1995

CHAPTER OF ELECTROLYSIS

H. M .FAHMY

1-         Electrolysis of an NaCl solution for 24 hours with a current of 15500A produced 4200 liter of cell liquor containing 125g/l of NaOH.  Calculate the efficiency of this process.

2-         A 10X10 cm metal plate was electroplated with 50µm nickel in an electrolyte solution containing Ni²⁺ ions.  Calculate the time needed to produce this electroplating if a current of 2.5A was used knowing that the yield by current is 94.4%.Sp.g. of Ni = 8.9

3-         Calculate the time of electrolysis of water to produce a mixture of .0325g of hydrogen and .2591g of oxygen.

4-         Calculate the current efficiency and the energy consumption in the electrolysis of a KCl solution.  The cathode compartment contains 500 ml of KCl solution.  After the electrolysis the Cu cathode of an in series coulometer increased in mass by 5g.  In assaying the KOH concentration in the cathodic compartment 25 ml of O.1N HCl were consumed for 10 ml of catholyte.  The voltage was controlled at 4.5V.

5-         Calculate the limiting current density for an electrode at which Ag is deposited from an aqueous solution containing Ag+ of 0.1 mol / l  in the presence of excess supporting electrolyte.  The diffusion layer = 0.05 cm and D= 1.64 X 10^-5 cm² / sec .  What well happen to the limiting current if the solution is rapidly stirred.

6-         It was found recently that pyrazolo[5,1- c]triazine (1) is reduced in 2 N HCI to give the hyrazo product in 70% yield. The following data was obtained for the electrochemical reduction of 10^-4 M of 1 (M.Wt 124) in acid medium.

(a) Coulometry: the potential was controlled at -1.0 V vs SCE the initial current i_o = 4 mA and k = 2 were obtained from log i_t vs log t plot.

(b)  dE _1/2 / dpH = 59 mV/pH

(c)  i_d at 25°C equals to 0.5 m A (m^3/4 t^1/6 = 1.16).

(d) C.V. of 1 showed two peaks at DEp 0.85 V with a current ip twice that of a one electron transfer.

From the above data propose a mechanism using your course knowledge, and calculate the diffusion and transfer coefficients.

7-         On the electroreduction of 5-arylidene-2thiohydantoins (T). In acid medium of pH < 7 the parent compound (X = H) showed one wave of il = 0.6mA (10^-4 M). the obtained productss of electrolysis at pH 3 where TH2 and the dimer 2TH in 65% and 35% yield. Propose the mechanism of electroreduction according to a square diagram.

8-         For an ideal CPE the relation i_t = i_o e^-kt is applied and the total quantity of electricity. Q is given by

Q = ò  i_t dt.

 (a) Show that Q = i / k (if natural logarithm is used).

 (b)  Q = i_o / 2.303 k if base-ten logarithm is used.

 (c) CPE oxidation of Fe (II) to Fe (III) in 1 M H₂SO₄ at a Pt anode at +0.8V vs SCE gives the following data:

          i, mA              t, m.          i, mA

          188                10             52.5

          164                12             39.4

          123                14             29.7

           92.8              16             22.3

construct a plot and calculate the number of mg of iron in the sample solution.

9-         Vitamin C tablets, advertised to each contain 120 mg of ascorbic acid, were analyzed by CPE coulometry. A single tablet was dissolved in 100 ml of a solution of a 0.1 M buffer of pH 6.03 with 0.25% oxalic acid, (to inhibit the autooxidation of ascorbic acid) this solution was electrolyzed at +1.09 V vs SCE according:

C₆H₆O₆  ® C₆H₆O₆   +   2H⁺  +   2e

The quantity of electricity was found to be 128.9 coulombs.  Calculate the weight of ascorbic acid in the tablet. Comment.

10-       Consider the reaction [ A + ne ® B ] where a+b = l. The flux at the electrode

J _{A (O , t)}  is given by the Butler-Volmer equation.

J_{A(O ,t)} = i / nFA = C_{A (O ,t)}  k°e ^{-a  n F(E-Eo) / RT}    -  C_{B ( O , t )} k°e^{(1 -a ) n F (E-Eo) /RT}.

If k^o  >  10^-1 cm s^-1 what would be the form of this equation.

If k^o < 10^-5 cm s^-1. Suggest the forms of the respective Cyclic Voltammograms.

11-       A log plot for a species undergoing polarographic reduction gives a slope of 0.03 (dE / d log [i / id-i ] ). (a) is the process reversible in nature; (b) knowing that dE_1/2 / dpH equals 60 mV pH^-1. Calculate the number of protons involved in this process; (c) assume a square diagram for the reduction of this species at pH<5.

12-       Answer three of the following including "a"(EXAM 1986)

-a- The reversibility of an electrode process is tested by the logarithmic analysis and cyclic voltammetry. Comment.

-b- "Nicholson-Shain predicted relations between various characteristics of cyclic voltammograms and scan rate for a given electrode mechanism". Give the 8 predicted cases referred to their diagrams.

-c- Draw the square diagram for a 2e process at pH 9.

-d- Give short notes on 2 only.

(i) Cyclic voltammetry of aniline.

(ii) The use of Q =òidt to determine e in an EC reaction .

(iii) Types of currents in polarography.

13-       Answer the following question (EXAM 1988)

i- Give short notes on one of the following: a-E.C. mechanisms in C.V. b-Multielectron transfer process in C.V.

ii- What would be the form of the Butler-Volmer equation in a simple E mechanism in C.V. if k^o ³ 10^-l cm s^-l. Draw its cyclic voltammogram.. iii- A log plot gives a slope of 0.03 Is the process reversible in nature . Knowing that dE / dpH equals 60 mV/ pH . Calculate The number of protons involved in this process. Assume a square diagram for the reduction process at pH< 5.

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