CBSE Class 12 Chemistry 2020 Outside Delhi Set 1 Solved Paper

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Question : 35 of 37
 
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SECTION -D

(a) The electrical resistance of a column of 0.05 MKOH solution of length 50cm and area of cross-section 0.625cm2 is 5×103ohm. Calculate its resistivity, conductivity and molar conductivity.
(b) Predict the products of electrolysis of an aqueous solution of CuCl2 with platinum electrodes.
(Given : E°Cu2+∕Cu=+0.34V, E°(1∕2Cl2∕Cl−)=+1.36V;
E°H+∕H2(g),Pt=0.00V, E°(1∕2O2∕H2O)=+1.23V)
OR
(a) Calculate e.m.f. of the following cell :
Zn( s)∕Zn2+(0.1M)∥ (0.01M)Ag+∕Ag( s)
Given : E°Zn2+∕Zn=−0.76V, E°Ag+∕Ag=+0.80V
[Given : log10=1]
(b) X and Y are two electrolytes. On dilution molar conductivity of ' X ' increases 2.5 times while that Y increases 25 times. Which of the two is a weak electrolyte and why?
Solution:
(a) Given : A=0.625cm2,l=50cm
R=5×103ohm,ρ=?
m=0.05m,K=?
∧m=?
Cell constant =ℓA=50cm0.625cm2=80cm−1
Resistivity =R cell constant or R×Al
⇒5×103×0.62550
⇒5×103 ohm 80cm−1⇒62.5 ohm cm
⇒62.5 ohm cm.
Conductivity =1 Resistivity ×lA
15×103×500.625 =505×103×625×10−3
=10625=0.016 scm−1
Molar conductivity (Λm)=103KM
Λm=KM×1000=10×1000625×0.05=320 sm2mol
(b) Given: E°Cu2+∕Cu+0.34V
E°(1∕2Cl2∕Cl−)=+1.36V
E°H+∕H2(g)′Pt=0.00V,E°(1∕2O2∕H2O)=+1.23V
At cathode:
Cu(aq)2++2e−→Cu( s);E∘=0.34
H(aq)++e−→12H2(g);E∘=0.000V
The reaction with a higher value of E∘ takes place at the cathode, so deposition of copper will take place at the cathode.
At anode: The oxidation reactions are possible at the anode.
Cl(aq)−→12Cl2(g)+e−;E∘=1.36V
2H2O(l)→O2(g)+4H+(aq)+4e−; E∘=+1.23V
At the anode the reaction with a lower value of E∘ is preferred. But due to the over potential of oxygen, Cl−gets oxidised at anode to produce Cl2 gas.
OR
(a)
Zn( s)∕Zn2+(0.1M)∥ (0.01M)Ag+∕Ag( s)
E°Zn2+∕Zn=−0.76V
E°Ag+∕Ag=+0.80Vemf=?
Ecell =E°cell −0.0591nlog [Anode ][ Cathode ]
E°cell =E°cathode −E°anode
=E°Ag∕Ag−E°Zn∕Zn
=0.80−(−0.76)=1.56V
Ecell =1.56−0.05912log[Zn2+][Ag+]2
=1.56−0.05912log[0.1][0.01]2
=1.56−0.0295log1000
=1.56−3(0.0295)
=1.56−0.09=1.4715
(b) ϒ is a weak electrolyte as n dilution complete dissociation of weak electrolyte takes place and thus a sharp increase in molar conductivity while in strong electrolyte it has already dissociated completely. So on dilution molar conductivity does not rises much.
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