November 12, 2012

Physical Problems from Electro Chemistry - I

Q No. 70.d
1. What is the pH of a solution containing 0.5 M propionic acid and 0.5 M sodium propionate? The Ka of propionic acid is 1.34 x 10-5.
Ka of propionic acid = 1.34 x 10–5
pKa = – log Ka = – log (1.34 x 10–5)
           = 5 – log 1.34
           = 5 – 0.1271
    pKa = 4.8729
pH = pKa + log [salt] / [acid] ... Henderson - Hasselbalch equation
       = 4.8729 + log 0.5 / 0.5
 pH = 4.8729
Or
The dissociation equilibrium of propionic acid will be
C2H5COOH   C2H5COO + H+
Ka = [C2H5COO–][ H+ ] / [C2H5COOH ]
      = 0.5 x [ H+] / 0.5
Ka = [H+]
  pH = – log [H+] = – log Ka
           = – log (1.34 x 10–5)
           = 5 – log 1.34
           = 5 – 0.1271
   pH = 4.8729
2. Find the pH of a buffer solution containing 0.30 mole per litre of CH3COONa and 0.15 mole per litre of CH3COOH. Ka for acetic acid is 1.8 x 10-5.
Ka = 1.8 x 10–5
pKa = – log Ka 
pKa = – log (1.8 x 10–5)
        = 5 – log 1.8
        = 5- 0.2553
 pKa = 4.7447
pH = pKa + log[salt] / [acid] ... Henderson - Hasselbalch equation
= 4.7447 + log 0.30 / 0.15
pH = 4.7447 + log 2
= 4.7447 + 0.3010
pH = 5.0457
3. Calculate the pH of the buffer solution containing 0.04 M NH4Cl and 0.02 M NH4OH. For NH4OH Kb is 1.8 x 10-5.
Kb = 1.8 x 10–5
pKb = – log Kb 
pKb = – log (1.8 x 10–5)
        = 5 – log 1.8
        = 5- 0.2553
 pKa = 4.7447
pOH = pKb + log[salt] / [base] ... Henderson - Hasselbalch equation
         = 4.7447 + log 0.04 / 0.02
pOH = 4.7447 + log2
         = 4.7447 + 0.3010 = 5.0457
pH + pOH = 14.00
pH = 14.00 – pOH
pH = 14.00 – 5.0457
pH = 8.9543
4. Equivalent conductivity of acetic acid at infinite dilution is 390.7 and for 0.1 M acetic acid the equivalent conductance is 5.2 mho.cm2.gm.equiv–1. Calculate degree of dissociation, H+ ion concentration and dissociation constant of the acid.
Equivalent conductivity of acetic acid at infinite dilution, λ= 390.7
Equivalent conductance of acetic acid, λc = 5.2 mho.cm2.gm.equiv–1
Concentration of acetic acid, C = 0.1 M
Degree of dissociation,  a = λc / λ
                                          = 5.2 / 390.7
                                           = 0.01333
                                            = 1.33 x 10-2 = 1.33%
CH3COOH H+ + CH3COO
C (1 – a)           Ca    Ca
  H+ ion concentration, [H+] = Ca
                                                = 0.1 x 0.0133
                                                = 0.00133
                                                = 1.33 x 10-2 M
 Dissociation constant of acetic acid, K = a2C / 1 - a
                                                              = (0.0133)2 x 0.1 / (1 - 0.0133)
                                                              = 0.000017689 / 0.9867
                                                              = 1.79 x 10-5 M
5. Calculate / Find the pH of a buffer solution containing 0.2 / 0.20 mole per litre CH3COONa and 0.15 mole per litre CH3COOH. Ka for acetic acid is 1.8x10-5.
Ka = 1.8 x 10–5
pKa = – log Ka 
pKa = – log (1.8 x 10–5)
        = 5 – log 1.8
        = 5- 0.2553
 pKa = 4.7447
pH = pKa + log [salt] / [acid] ... Henderson - Hasselbalch equation
      = 4.7447 + log 0.20 / 0.15
pH = 4.7447 + log 4 / 3
pH = 4.7447 + log 4 -  log 3
pH = 4.7447 + 0.6021 – 0.4771
pH = 4.8697
6. An electric current is passed through three cells in series containing respectively solutions of copper sulphate, silver nitrate and potassium iodide. What weights of silver and iodine will be liberated while 1.25 g of copper is being deposited?
Wt. of Copper / Wt. of Iodine        =  Eqvt. wt. Copper (31.7) / Eqvt. wt. of Iodine (127)
                                           1.25 / x = 31.7 / 127
                                Wt. of Iodine, x = 1.25 x 127 / 31.7
Hence,                    Wt. of Iodine, x = 5.0 g
Also, Wt. of Copper / Wt. of Silver = Eqvt. wt. of Copper (31.7) / Eqvt. wt. of Silver (108)
                                             1.25 / y = 31.7 / 108
                                  Wt. of Silver, y = 108 x 1.25 / 317
                                Wt. of Silver, y = 4.26 g
Or
31.7 g of Copper (1g eqvt) is liberated by = 96,495 coulomb
1.25 g of Copper is liberated by = 96,495 x 1.25 / 31.7 coulomb
Quantity of electricity = 3805 coulomb
96,495 coulomb deposits 127 g of Iodine
3805 coulomb deposits = 127 x 3805 / 96,495
     = 5.0 g of Iodine
96,495 coulomb deposits 108 g of Silver
3805 coulomb deposits = 108 x 3805 / 96,495
     = 4.26 g of Silver
7. The equivalent conductance of HCl, CH3COONa and NaCl at infinite dilution are 426.16, 91.0 and 126.45 ohm-l cm2 (gram equivalent)-1 respectively. Calculate the equivalent conductance / λof acetic acid.
λCH3COOH = λCH3COONa + λHCl – λNaCl
λCH3COOH = 91.0 + 426.16 – 126.45 = 517.16 - 126.45
λCH3COOH = 390.71
8. 0.1978 g of copper is deposited by a current of 0.2 ampere in 50 minutes. What is the electrochemical equivalent of copper?
Current, I = 0.2 ampere
Time, t = 50 minutes = 50 ´ 60 seconds
Quantity of electricity used, Q = I x t = 0.2 x 50 x 60 = 600 coulombs
Amount of copper deposited by 600 coulombs, m = 0.1978 g
Amount of copper deposited by 1 coulomb, Z = 0.1978 / 600g = 0.0003296 g
Electrochemical equivalent of copper, Z =  m  /  I . t
                                                             =0.0003296 gC–1
                                                             = 3.296 x 10–4 gC–1
                                                             = 3.206 x 10–7 kgC–1
Or
Electrochemical equivalent of copper, Z =  m / I . t
                                                   (or) Z =  m / Q
                                                             = 0.1978 g / 0.2 amp x 50 x 60 sec
                                                             = 0.1978 g / 600 C       [ amp . sec = C
                                                             =0.0003296 = 3.296 x 10–4 gC–1
                                                             = 3.296 x 10–7 kgC–1
9. Calculate the pH of 0.1 M acetic acid / CH3COOH solution. Dissociation constant of acetic acid is 1.8 x 10-5 M.
For weak acids, [H+] = Ka x C
                                  = 1.8 x 10-5 x 0.1 = 1.8 x 10-6
                                  = 1.34 x 10–3 M
pH = – log [H+]
          = – log (1.34 x 10–3)
          = 3 – log1.34
          = 3 – 0.1271
 pH = 2.8729
10. 0.04 N solution of a weak acid has a specific conductance 4 x10–4 mho.cm–1. The degree of dissociation of acid at this dilution is 0.05. Calculate the equivalent conductance of weak acid at infinite solution.
Specific conductance, κ = 4 x 10-4 mho.cm-1
Equivalent conductance of weak acid, λc = κ .1000 / C
                                                                    = 4 x 10-4 x 1000 / 0.04
                                                               λc = 10 mho.cm2.eq-1
Degree of dissociation, α = 0.05

α = λc / λ
Equivalent conductance of weak acid at infinite solution, λ∞ = λc / α
  λ= 10 / 0.05
λ= 200 mho.cm2.gm.equiv.-1
11.Ionic conductances at infinite dilution of Al3+ and SO42– are 189 ohm–1cm2gm.equiv-1 and 160 ohm-1cm2gm.equiv.-1. Calculate equivalent and molar conductance of the electrolyte at infinite dilution.
Equivalent conductance of the electrolyte at infinite dilution,
λ= 1/n+. λA+ + 1/m- . λB 
Where,
λ+ and λ are the cationic and anionic equivalent conductances at infinite dilution
n+ and m correspond the valency of cations and anions
The electrolyte is Al2(SO4)3
λAl2(SO4)3 = 1/3 λ∞ Al3+ + 1/2 λ∞ SO42– 
λAl2(SO4)3 = 189 / 3 + 160 / 2
                        = 63 + 80
                         = 143 mho cm2 gm.equiv–1
Molar conductance of the electrolyte at infinite dilution,
μ = γ+ μ+ + γ- μ-
Where,
μ- and μ+are the ionic conductances at infinite dilution
γ+ =  number of cations and γ- = number of anions
μAl2(SO4)3 = 2 μ+ + 3 μ-
                         = (2 x 189) + (3 x 160)
                         = 378 + 180
                         = 858 mho cm2 mol–1
12. What current strength in amperes will be required to liberate 10 g of iodine from potassium iodide solution in one hour?
127 g of iodine (1g eqvt) is liberated by = 96,495 coulomb
10 g of iodine is liberated by = (96,495 x 10 / 127) coulomb
Let the current strength be = I
Time (in seconds) = 1 x 60 x 60 = 3600 seconds
The quantity of electricity used, Q = I x Time (in seconds)
Curernt strength, I = Q / t
                              = 96,495 coulomb x 10 / 127 x 3600 seconds
                              = 2.11 coulomb . second–1                             
 or                         = 2.11ampere         [ coulomb . second–1 = ampere

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