Lakshya Education MCQs

Question: If concentration of OHions in the reaction  Fe(OH)3(s)  Fe3++(aq)+ 3 OH (aq) is decreased by 1/4 times, then equilibrium concentration of Fe3+will increase by
Options:
A.8 times
B.16 times
C.64 times
D.4 times.
Answer: Option C
: C


K=[Fe3+][OH]3 If concentration of OHion is decreased to 14 th to keep K constant
Concentration of Fe3+ions has to be increased by 64 times.

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Question 1. The values of Kp1and KP2 for the reactions

X  Y + Z and A 2B Are in the ratio of 9:1. If the degree of dissociation of X and A be equal, then total pressure at equilibrium (1) and (2) are in the ration
  1.    3:1
  2.    1:9
  3.    36:1
  4.    1:1
Answer: Option C
: C


Suppose total pressure at equilibrium for reactions 1 and 2 areP1andP2respectively

XY+ZInitialmoles100Atequilibrium1xxx

Totalmoles=1+α
Partial pressure of X=px=(1x)XP11+x
Py<=(x)XP11+x=Pz
(KP1=PYPZ/Px=x2P1/(1x2)x2P1

A2BhlineInitialmoles10Atequilibrium1x2x

Totalmoles=1+α

PA=(1x)XP21+xPB=(2x)XP21+x

KP2=P2B/PA=4x2P2/(1x2)4x2P2
KP1/KP2=x2P1/4x2P2=9/1
P1/P2=36/1
Question 2. For a hypothetical reaction

4A + 5B 4P+6Q,

The equilibrium constant  Kc has units
  1.    molL−1
  2.    mol−1L
  3.    (molL−1)−2
  4.    unit less   
Answer: Option A
: A


Kc=[P]4[Q]6/([A]4[B]5)=(molL1)10/(molL1)9/=molL1
Question 3. Equimolar concentrations of H2 and I2 are heated to equilibrium in a 2litre flask. At equilibrium, the forward and backward rate constants are found to be equal. What percentage of initial concentration of H2has reacted at equilibrium?
  1.    33.33%
  2.    75%
  3.    50%
  4.    15%
Answer: Option C
: C

Solution:Correct option is c).

H2(g)+l2(g)2HI(g)Initialmoles110Atequilibrium1x1x2xMolarconcentration(mol/L)(1x)/2(1x)/22x/2=x

K=x2/(1x)×(1x)2×2=4x2/(1x)24x2
K=Kf/Kb=1

4x2=1x=12,i.e.,50%
Question 4. At 30C,Kp for the dissociation reaction:

SO2Cl2 (g) SO2 (g) + Cl2 (g) is 2.9x 102~atm. If the total pressure is 1 atm, the degree of dissociation of SO2Cl2 is (assume 1α2 =1)
  1.    85%
  2.    12%
  3.    17%
  4.    35%
Answer: Option C
: C

Correct option is c).
SO2Cl2(g)SO2(g)+Cl2(g)1001ααα
Total moles = 1+α
Pso2Cl2=1α/1+α,Pso2=α/1+α,Pcl2=α/1+α
Kp=(α/1+α)2/(1α/1+α)=α2/1α2α2
α=Kp=(2.9×102=0.17,i.e.,17%
Question 5. 1 mole of N2 O4 (g) at 300K is kept in a closed container under 1 atm pressure. It is heated to 600K when 20% by mass of N2O4 (g) decomposes to NO2 (g) . The resultant pressure is
  1.    1.2 atm
  2.    2.4 atm
  3.    2.0 atm
  4.    1.0 atm
Answer: Option B
: B

Correct option is b).
N2O42NO2Initialmoles10

Total moles=0.8+0.4=1.2

Applying, PV=nRT, at constant volume,

P1n1T1=Pn2T2

11X300=P21.2X600

P2=2.4atm.
Question 6. The vapor density of completely dissociated NH4Cl would be
  1.    same as that of NH4Cl
  2.    double than that of NH4Cl
  3.    half than that of NH4Cl
  4.    slightly less than that of NH4Cl
Answer: Option A
: A


On dissolution of NH4Cl into NH3+HCl number of moles become double. Hence, volume is doubled and therefore, density is halved.

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