Wbjee 2018 Physics Chemistry Solution
Wbjee 2018 Physics Chemistry Solution
Wbjee 2018 Physics Chemistry Solution
Code -
1. Four resistors 100 , 200 , 300 and 400 are connected to form four sides of a square. The resistors can be
connected in any order. What is the maximum possible equivalent resistance across the diagonal of the square ?
(A) 210 (B) 240 (C) 300 (D) 250
Ans : (D)
R2 B
Hint : R1 R4
A R3
(R1 R 2 )(R3 R 4 )
R AB . As Ri const., for RAB = maximum, R1 + R2 = R3 + R4
(R1 R2 R3 R4 )
The only possible combination is 100 + 400 = 200 + 300
2. What will be current through the 200 resistor in the given circuit a long time after the switch ‘K’ is made on ?
100 1F 400
6V K
(A) Zero (B) 100 mA (C) 10 mA (D) 1 mA
Ans : (C)
Hint : Arms having capacitances will not conduct in steady state
WBJEE - 2018 (Answers & Hint) Physics & Chemistry
200 400
6
i 10mA
600
3. A point source is placed at co-ordinates (0, 1) in X-Y plane. A ray of light from the source is reflected on a plane mirror
placed along the X-axis and perpendicular to the X-Y plane. The reflected ray passes through the point (3, 3). What is
the path length of the ray from (0,1 ) to (3, 3) ?
(0,1) (3, 3)
O
Hint :
(0,–1)
f f
(A) F > f (B) F f (C) F (D) F=f
2 2
Ans : (B)
1 1 1 1 ff
Hint : f f f f , F
eq f 2f
f f
F F and F f
2 f / f 2
5. There is a small air bubble at the centre of a solid glass sphere of radius ‘r’ and refractive index ‘’. What will be the
apparent distance of the bubble from the centre of the sphere, when viewed from outside ?
r 1
(A) r (B) (C) r r (D) Zero
Ans : (D)
Hint : As the object is at centre, all rays will fall normally on surface, hence will not deviate.
6. If Young’s double slit experiment is done with white light, which of the following statements will be true ?
(A) All the bright fringes wil be coloured.
(B) All the bright fringes will be white
(C) The central fringe will be white
(D) No stable interference pattern will be visible
Ans : (C)
Hint : The central fringe will be white as x = 0 at central bright fringe which is independent of .
7. How the linear velocity ‘v’ of an electron in the Bohr orbit is related to its quantum number ‘n’ ?
1 1 1
(A) v (B) v (C) v (D) vn
n n2 n
Ans : (A)
nh nh 1
Hint : mvr = , mv.r0n2 = , v
2 2 n
8. If the half life of a radioactive nucleus is 3 days, nearly what fraction of the initial number of nuclei will decay on the 3rd
3
day ? (Given that 0.25 0.63 )
(A) 0.63 (B) 0.5 (C) 0.37 (D) 0.13
Ans : (D)
N0
Hint : N t1/ 2 3 days
2t/ t1/2
N0 N0
At t = 2 days; N1 2/3
1/3 0.63N
2 4
N0 N N2 (0.63 0.5)N0
At t = 3 day, N2 = , Fraction disintegrated = 1 = 0.13
2 N0 N0
9. An electron accelerated through a potential of 10,000 V from rest has a de-Broglie wave length ‘’. What should be the
accelerating potential so that the wave length is doubled ?
(A) 20,000 V (B) 40,000 V (C) 5,000 V (D) 2,500 V
Ans : (D)
1
Hint : As
V
V 1 10000
, or V 2500 V
V 4 4
10. In the circuit shown, inputs A and B are in states ‘1’ and ‘0’ respectively. What is the possible stable state of the
outputs ‘X’ and ‘Y’ ?
A
(1)
X
(0)
B Y
(A) X = ‘1’, ‘Y’ = ‘1’ (B) X = ‘1’, ‘Y’ = ‘0’ (C) X = ‘0’, ‘Y’ = 1 (D) X = ‘0’, ‘Y’ = ‘0’
Ans : (C)
WBJEE - 2018 (Answers & Hint) Physics & Chemistry
A=1
Y
X
Hint : X AY Y , Y BX 1, X = 0
X
B=0
Y
11. What will be the current flowing through the 6 K resistor in the circuit shown, where the breakdown voltage of the
zener is 6 V ?
6 K
10V 4 K
2 3
(A) mA (B) 1 mA (C) 10 mA (D) mA
3 2
Ans : (B)
Hint : Since potential difference cross zener diode is 4 which is less than its breakdown voltage, therefore it does
not conduct.
10
i 1 mA
(6 4) 103
12. In case of a simple harmonic motion, if the velocity is plotted along the X-axis and the displacement (from the
equilibrium position) is plotted along the Y-axis, the resultant curve happens to be an ellipse with the ratio :
major axis (along X)
20
minor axis (along Y)
What is the frequency of the simple harmonic motion ?
1
(A) 100 Hz (B) 20 Hz (C) 10 Hz (D) Hz
10
Ans : (C)
A
Hint : 20
A
= 20 = f.2 f = 10 Hz
13. A block of mass m2 is placed on a horizontal table and another block of mass m1 is placed on top of it. An increasing
horizontal force F = t is exerted on the upper block but the lower block never moves as a result. If the co-efficient of
friction between the blocks is 1 and that between the lower block and the table is 2, then what is the maximum
possible value of 1/2 ?
m2 m2 m1 m1
(A) (B) 1 (C) (D) 1
m1 m1 m2 m2
Ans : (B)
Hint : F = t m1 1
m2 2
1 m
2 (m1 + m2) g 1 m1 g, 1 2
2 m1
WBJEE - 2018 (Answers & Hint) Physics & Chemistry
14. In a triangle ABC, the sides AB and AC are represented by the vectors 3i j k and i 2j k respectively. Calculate
the angle ABC
5 6 1 5 1 5
(A) cos–1 11 (B) cos–1 (C) 90 cos (D) 180 cos
11 11 11
Ans : (A)
Hint :
A C
AB 3i j k , AC i 2j k , AB BC AC BC AC AB = 2i j ABC = Angle between BA and BC
where BA = 3i j k BA AB
cos1 BA BC 6 1 5 5
= = cos–1 2 2 2 = cos–1 = cos–1
BA BC 3 1 1 22 12 11 5 11
1
15. The velocity (v) of a particle (under a force F) depends on its distance (x) from the origin (with x > 0) v . Find how
x
the magnitude of the force (F) on the particle depends on x
1 1 1
(A) F 3 (B) F (C) F (D) F x
2
x x2
x
Ans : (C)
1 K
Hint : v v=
x x
vdv K K 3 2 K 2 1
F x = ,F 2
dx x 2 2x 2
x
16. The ratio of accelerations due to gravity g1 : g2 on the surfaces of two planets is 5:2 and the ratio of their respective
average densities 1 : 2 is 2 : 1. What is the ratio of respective escape velocities v 1 : v 2 from the surface of the
planets ?
(A) 5:2 (B) 5: 2 (C) 5:2 2 (D) 25 : 4
Ans : (C)
g2 v1 25 5
Hint : v e = 2gR , v e v2 = 8 = 2 2
,
17. A spherical liquid drop is placed on a horizontal plane. A small disturbance causes the volume of the drop to oscillate.
The time period of oscillation (T) of the liquid drop depends on radius (r) of the drop, density () and surface tension (s)
of the liquid. Which among the following will be a possible expression for T (where k is a dimensionless constant) ?
r 2r r 3 r 3
(A) k (B) k (C) k (D) k
s s s s2
Ans : (C)
WBJEE - 2018 (Answers & Hint) Physics & Chemistry
Hint : T = krabsc, T = K[L]a [ML-3]b [MT–2]c , [M°L° T1] = K[M]b+c [L]a–3b [T]–2c
–2c = 1 b+c=O a – 3b = 0
c = 12 b = –c a = 3b
b = 12 a = 3/2
r 3
T = Kr3/2 1/2 s–1/2 = T = K
s
18. The stress along the length of a rod (with rectangular cross section) is 1% of the Young’s modulus of its material.
What is the approximate percentage of change of its volume ? (Poisson’s ratio of the material of the rod is 0.3)
(A) 3% (B) 1% (C) 0.7% (D) 0.4%
Ans : (D)
F 1 F L F / A 1
Hint : Y,Y= A ,
A 100 L Y 100
L
r/r
V =–
= 2 r + L L/L
V r L – r/r
0.3 =
1/100
0.3 1 0.3
=2× = r r
100 100 100
0.4
= = 0.4%
100
19. What will be the approximate terminal velocity of a rain drop of diameter 1.8×10–3 m, when density of rain water 103
kgm—3 and the co-efficient of viscosity if air 1.8 × 10–5 Nsm–2 ? (Neglect buoyancy of air)
(A) 49 ms–1 (B) 98 ms–1 (C) 392 ms–1 (D) 980 ms–1
Ans : (B)
21. One mole of a mono-atomic ideal gas undergoes a quasi-static process, which is depicted by a straight line joining
points (V0, T0) and (2V0, 3T0) in a V-T diagram. What is the value of the heat capacity of the gas at the point
(V0, T0) ?
3
(A) R (B) R (C) 2R (D) 0
2
Ans : (C)
P dV 3R P Vo 3R R
Hint : Cprocess = Cv + = 2 n 2T = 2R
n dT o 2 2
22. For an ideal gas with initial pressure and volume Pi and Vi, respectively, a reversible isothermal expansion happens,
when its volume becomes V0. Then it is compressed to its original volume Vi by a reversible adiabatic process. If the
final pressure is Pf, then which of the following statements is true ?
Pf P
(A) Pf = Pi (B) Pf > Pi (C) Pf < P i (D) i
V0 Vi
Ans : (B)
Pf
Hint :
Pi
V
Vi Vo
23. A point charge –q is carried from a point A to another point B on the axis of a charged ring of radius ‘r’ carrying a charge
4 3
+q. If the point A is at a distance r from the centre of the ring and the point B is r from the centre but on the
3 4
opposite side, what is the net work that need to be done for this ?
7 q2 1 q2 7 q2 1 q2
(A) (B) (C) (D)
5 4or 5 4or 5 4or 5 4or
Ans : (B)
A B 5r
A =
3
5r
Hint : A 4r B B =
3r 4
3 4
2
Kq Kq 4 3 Kq2 1 q
W = –q (VB – VA) = –q = –qKq
5r 5r = = 5 4 r
B A 5r o
WBJEE - 2018 (Answers & Hint) Physics & Chemistry
24. Consider a region in free space bounded by the surfaces of an imaginary cube having sides of length ‘a’ as shown in
the diagram. A charge +Q is placed at the centre ‘O’ of the cube. P is such a point outside the cube that the line OP
perpendicularly intersects the surface ABCD at R and also OR = RP = a/2. A charge +Q is placed at point P also.
What is the total electric flux through the five faces of the cube other than ABCD ?
A
a
B
a/2 a/2
O P
a +Q R D +Q
a C
Q 5Q 10Q
(A) 0
(B) 60
(C) 6 0
(D) zero
Ans : (A)
Q
Hint : No net flux with ABCD and net flux through entire cube = .
o
25. Four equal charges of value +Q are placed at any four vertices of a regular hexagon of side ‘a’. By suitably choosing
the vertices, what can be the maximum possible magnitude of electric field at the centre of the hexagon ?
Q 2Q 3Q 2Q
(A) (B) (C) (D)
40 a2 40 a2 40 a2 40 a2
Ans : (C)
Hint :
E +Q
E
E
E
+Q +Q
+Q
E Enet 3 E
E
60° 3 Kq 3q
=
a 2
4o a2
26. A proton of mass ‘m’ moving with a speed v (<< c, velocity of light in vacuum) completes a circular orbit in time ‘T’ in
a uniform magnetic field. If the speed of the proton is increased to 2 v, what will be time needed to complete the
circular orbit ?
T T
(A) 2T (B) T (C) (D)
2 2
Ans : (B)
Hint : Time period is independent of velocity. So new time period = T
WBJEE - 2018 (Answers & Hint) Physics & Chemistry
27. A uniform current is flowing along the length of an infinite, straight, thin, hollow cylinder of radius ‘R’. The magnetic
field ‘B’ produced at a perpendicular distance ‘d’ from the axis of the cylinder is plotted in a graph. Which of the
following figures look like the plot ?
B
B
(A) (B)
d
d R
R
B B
(C) (D)
d d
R R
Ans : (C)
Hint : B = 0 d<R
1
B dR
d
28. A circular loop of radius ‘r’ of conducting wire connected with a voltage source of zero internal resistance produces a
magnetic field ‘B’ at its centre. If instead, a circular loop of radius ‘2r’, made of same material, having the same cross
section is connected to the same voltage source, what will be the magnetic field at its centre ?
B B
(A) (B) (C) 2B (D) B
2 4
Ans : (B)
0 V V
Hint : B , B' 0 , , '
2r 2.2r R 2R 2
B
B' 0
4x2r 4
29. An alternating currect is flowing through a series LCR circuit. It is found that the current reaches a value of 1 mA at
both 200 Hz and 800 Hz frequency. What is the resonance frequency of the circuit ?
(A) 600 Hz (B) 300 Hz (C) 500 Hz (D) 400 Hz
Ans : (D)
1 1 1 1
(A) B (B) B (C) B (D) B 5
r r2 3
r 2 r2
Ans : (D)
-q mv 2 KqQ mv 2
F F 2 m2r
r r r
1
3
Hint : Q r 2
q 1 0 i 1
equivalent current, i 3 , B 5
2 r 2 2r r 2
32. As shown in the figure, a rectangular loop of a conducting wire is moving away with a constant velocity ‘v’ in a
perpendicular direction from a very long straight conductor carrying a steady current ‘I’. When the breadth of the
rectangular loop is very small compared to its distance from the straight conductor, how does the e.m.f. ‘E’ induced
in the loop vary with time ‘t’ ?
1 1 1
(A) E (B) E (C) E 1n(t) (D) E
t2 t t3
Ans : (A)
E2 = vB2l
Hint :
l
b B2 v = constant
B1
y
E1 = vB1l
I
WBJEE - 2018 (Answers & Hint) Physics & Chemistry
Eloop E1 E2
vl B1 B2
0
vl 0
2y 2(y b)
b
vl 0
2 y y b
as b y,
1
Eloop 2
y
1
E100p
t2
33. A solid spherical ball and a hollow spherical ball of two different materials of densities 1 and 2 respectively have
same outer radii and same mass. What will be the ratio the moment of inertia (about an axis passing through the
centre) of the hollow sphere to that of the solid sphere ?
5 5
5 5
2 2 3 2 2 3 2 1 3 2 1 3
1 1 1 1
(A) 1 (B) 1 1 (C) 1 (D) 1 2
1 1 1 2
Ans : (D)
Hint :
r
1
P1
R
R
m
m
4 4
m 1 R3 2 (R3 r 3 )
3 3
3 1
3
r r
1 1 1 1
2 R R 2
2 4 2 4 3 2
2 R 3 R 2 2 r r
Ihollow 5 3 5 3
Isolid 2
mR2
5
r 5
R3 2 R2 1
R
4
3 mR2
4 R3 2 r 5
1
3 4 R
3
1 R
3
5 3
2 1 1 1
1 2
WBJEE - 2018 (Answers & Hint) Physics & Chemistry
34. The insulated plates of a charged parallel plate capacitor (with small separation between the plates) are approaching
each other due to electrostatic attraction. Assuming no other force to be operative and no radiation taking place,
which of the following graphs approximately shows the variation with time (t) of the potential difference (V) between the
plates ?
V V
(A) (B)
t t
0,0 0,0
V V
(C) (D)
t t
0,0 0,0
Ans : (A)
Hint :
2F 1 2
arel = , V = Ed - E do 2 arel t
m
parabolic with downward concavity
35. The bob of a pendulum of mass ‘m’, suspended by a inextensible string of length ‘L’ as shown in the figure carries a
small charge ‘q’. An infinite horizontal plane conductor with uniform surface charge density ‘’ is placed below it. What
will be the time period of the pendulum for small amplitude oscillations ?
m
+++++++++++++ ++++++++++++
L L L
1 L
(A) 2 (B) (C) (D) 2 q
mq mq 2 q
g g g
0 g 0 m
0 0m
Ans : (B,C,D)
Hint : apparent weight mg = mg – q E
q q L
mg = mg – , g = g – m , T = 2 q
0 0
g
0 m
WBJEE - 2018 (Answers & Hint) Physics & Chemistry
36. A non-zero current passes through the galvanometer G shown in the circuit when the key ‘K’ is closed and its value
does not change when the key is opened. Then which of the following statement(s) is/are true ?
200 300
K
G
100
10V
10v 10v
10 10
IG I3 300 G
100 G 100 G
G 1
I3
30 100 G
1 G 300 G 1
I3 + IG =
100 G 10 30 100 G 30
200
10
300G 300 G 1
3 300 G
100 G 30
6000 1100G 1 1
10 =
3 100 G 30
900 (100 + 6) = 60000 + 1100G
30000 = 200G
G = 150
WBJEE - 2018 (Answers & Hint) Physics & Chemistry
10 10
IG 40mA
100 150 250
200 300
As I200 = I300
100 150
37. A ray of light is incident on a right angled isosceles prism parallel to its base as shown in the figure. Refractive index
of the material of the prism is 2 . Then which of the following statement (s) is/are true?
Q
S
P
(A) The reflection at P is total internal
(B) The reflection at Q is total internal
(C) The ray emerging at R is parallel to the ray incident at S
(D) Total deviation of the ray is 150°
Ans : (A, C)
R 45°
30° 45°
15°
15° S
30°
Q 15°
75° 75°
15°
P
39. Which of the following statement(s) is/are true? “Internal energy of an ideal gas ___”
(A) Decreases in an isothermal process (B) Remains constant in an isothermal process
(C) Increases in an isobartic process (D) Decreases in an isobaric expansion
Ans : (B)
Hint : In isothermal process
T = constant
U = n CVT
In isobaric expansion [P = constant]
V T T increases due to expansion increase in internal energy
40. Two positive charges Q and 4Q are placed at points A and B respectively, where B is at a distance ‘d’ units to the right
of A. The total electric potential due to these charges is minimum at P on the line through A and B. What is (are) the
distance (s) of P from A?
d d
(A) units to the right of A (B) units to the left of A
3 3
d
(C) units to the right of A (D) d units to the left of A
5
Ans : (A)
Q E=0 4Q
A B
Hint :
x d-x
As V = min
V
0 E=0
r
kQ k 4Q d
x 2 d x 2 x 3
CATEGORY - I (Q41 to Q70)
Carry 1 mark each and only one option is correct. In case of incorrect answer or any combination of more
than one answer, ¼ mark will be deducted
44. The ease of hydrolysis in the compounds CH3COCl (I), CH3 – CO – O – COCH3 (II), CH3COOC2H5 (III) and CH3CONH2
(IV) is of the order
(A) I > II > III > IV (B) IV > III > II > I (C) I > II > IV > III (D) II > I > IV > III
Ans : (A)
Hint : Ease of hydrolysis is directly proportional to positive charge density on carbonyl carbon.
45. CH3 – C C MgBr can be prepared by the reaction of
(A) CH3 – C C – Br with MgBr2 (B) CH3 – C CH with MgBr2
(C) CH3 – C CH with KBr and Mg metal (D) CH3 – C CH with CH3MgBr
Ans : (D)
Hint : CH3 – C C – H is a terminal alkyne, which has acidic hydrogen.
–
CH3 – C C – H + CH3MgBr CH3C CMg Br CH4
46. The number of alkene (s) which can produce 2-butanol by the successive treatment of (i) B2H6 in tetrahydrofuran
solvent and (ii) alkaline H2O2 solution is
(A) 1 (B) 2 (C) 3 (D) 4
Ans : (B)
i) B2H6+ THF OH
–
cis-2-butene ii) H2O2/OH
Butan-2-ol
CH3
Cl
NH3 Br2
C8H6Cl2O C8H8ClNO
NaOH
(M)
H2N
O
O Cl
CHO
C
Cl C Cl
Cl Cl
(A) (B) (C) (D) C
Cl
Cl CH3
CH3 CH2Cl O
CH3
Ans : (B)
CH3 CH3
CH3
Cl Cl
Cl H2N
Br2/NaOH NH3 Cl
Hint : ( is Retrosynthetic arrow)
H2N O O
WBJEE - 2018 (Answers & Hint) Physics & Chemistry
+
O CH3 + HI O — CH3 + I–
|
H SN 2
OH + CH3 —I
K 2 Cr2 O7 I2 /NaOH
49. C 4H10 O
H2 SO4
C 4H8 O warm
CHI3 Here, N is
N
OH
(A) OH (B) (C) O (D) OH
Ans : (B)
OH K2Cr 2O 7 O I /NaOH O
2
Hint : + CHI 3
H2SO 4 warm – +
O Na (yellow)
50. The correct order of reactivity for the addition reaction of the following carbonyl compounds with ethylmagnesium
iodide is
H H3C H (CH3)3C
C O C O C O C O
H H3C H3C (CH3)3C
(I) (II) (III) (IV)
(A) I > III > II > IV (B) IV > III > II > I (C) I > II > IV > III (D) III > II > I > IV
Ans : (A)
Hint : Reactivity of carbonyl compounds with nucleophile depends on both steric and electronic factors
Reactivity order :
O O O O
51. If aniline is treated with conc. H2SO4 and heated at 200°C, the product is
(A) Anilinium sulphate (B) Benzenesulphonic acid
(C) m-Aminobenzenesulphonic acid (D) Sulphanilic acid
Ans : (D)
+ –
+
NH2 NH3 HSO4 NH2 NH3
+ H2SO4
Hint :
(–H2O)
SO3H SO3
–
WBJEE - 2018 (Answers & Hint) Physics & Chemistry
1
No of moles = 108
9.11 6.023
59. Equal weights of ethane and hydrogen are mixed in an empty container at 25°C. The The fraction of total pressure
exerted by hydrogen is
(A) 1:2 (B) 1:1 (C) 1:16 (D) 15:16
Ans : (D)
WBJEE - 2018 (Answers & Hint) Physics & Chemistry
m
PH2 2
Hint : 15 : 16
PT m m
2 30
60. The heat of neutralization of a strong base and a strong acid is 13:7 kcal. The heat released when 0.6 mole HCl
solution is added to 0.25 mole of NaOH is
(A) 3.425 kcal (B) 8.22 kcal (C) 11.645 kcal (D) 13.7 kcal
Ans : (A)
–
Hint : H+ + OH H2O
t=0 0.6 mol 0.25 mol Heat released = 0.25 × 13.7 = 3.421 kcal
Hint : X Y
1 1
8
8
=1
X:Y=1:1 XY
62. What amount of electricity can deposit 1 mole of Al metal at cathode when passed through molten AlCl3 ?
(A) 0.3 F (B) 1 F (C) 3 F (D) 1/3 F
Ans : (C)
Hint : Al+3 + 3e– Al
3F 1 mol
63. Given the standard half-cell potentials (E0) of the following as
Zn = Zn2+ + 2e– E0 = +0.76 V
Fe = Fe2+ + 2e– E0 = 0.41 V
Then the standard e.m.f. of the cell with the reaction Fe2+ + Zn Zn2+ + Fe is
(A) –0.35 V (B) +0.35 V (C) +1.17 V (D) –1.17 V
Ans : (B)
o o o
Hint : Ecell EFe2 /Fe – E Zn2 /Zn = –0.41 – (–0.76) = +0.35 V
64. The following equilibrium constants are given :
N2 + 3N2 2NH3; K1
N2 + O2 2NO; K2
1
H2 + O2 H2O; K3
2
The equilibrium constant for the oxidation of 2 mol of NH3 to give NO is
K2 K 33 K 23 K3
(A) K1 (B) K2 (C) K2 (D) K 22
K3 K1 K1 K1
Ans : (B)
WBJEE - 2018 (Answers & Hint) Physics & Chemistry
1 K2K33
Hint : 2NH3 N2 + 3H2 ; Kc
K1 K1
N2 + O2 2NO ; K2
3
3H2 + O2 3H2O ; K33
2
5 2NO + 3H2O
2NH3 + O2
2
20%H SO
Q
Hg ,
R
2
2 Zn Hg/HCl
4
S
Hint : H H Br H + –
Br2 Na NH2
C C H C C H H C C H
NH3(l) [Q]
H H H
[P] Br (E2) 2O% H2SO4
O Hg+2
72. The number of possible organobromine compounds, which can be obtained in the allylic bromination of 1 – butene
with N – bromosuccinimide is
(A) 1 (B) 2 (C) 3 (D) 4
Ans : (D)
Br CH3 H
CH2 C C
CH3 CH CH
* H CH2Br
(R and S) +
CH3 CH2 Br
C C
H H
73. A metal M (specific heat 0.16) forms a metal chloride with = 65% chlorine present in it. The formula of the metal
chloride will be
(A) MCl (B) MCl2 (C) MCl3 (D) MCl4
Ans : (B)
Hint : Using Dulong Petit’s law
At mass (approx) × sp. heat = 6.4
6.4
At. mass(approx) 40
0.16
Let formula is MClx
x 35.5
% by mass of Cl 100 65
40 35.5x
74. During a reversible adiabatic process, the pressure of a gas is found to be proportional to the cube of its absolute
Cp
temperature. The ratio for the gas is
Cv
3 7 5 9
(A) (B) (C) (D)
2 2 3 7
Ans : (A)
Hint : P T 3
or, P = KT3 or PT–3 = K ....... (i)
For reversible adiabatic process
1
TP K
K ........ (ii)
1
PT
Comparing equation (i) and equation (ii)
3
1
or = –3 + 3
3
or
2
(A) SO32 ,SO2 (B) Cl–, HCl (C) S2–, H2S (D) CO32 ,CO2
Ans : (A)
76. The possible product (s) to be obtained from the reaction of cyclobutyl amine with HNO2 is/are
O
OH CH2OH
(A) (B) (C) (D) H2C CH2
Ans : (A, C)
+
NH2 N2
+
HNO2
Hint :
OH
H2O –H+
+
+
Ring + H2O –H
CH2 CH2OH
contraction
H3C H
C C + Br2
77. The major product(s) obtained in the following reaction is/are
H C2H5
CH3 H
Hint : C C + Br2
H C2H5
Br C2H5
H Br
+
Br Path(a) CH3 H
H H Br
C C2H5
H
C2H5 CH3
C Br
(b)
CH3 H
–
Br
(a)
H
Path(b) C2H5 C2H5
Br
Br H
Br Br H
CH3 H CH3
WBJEE - 2018 (Answers & Hint) Physics & Chemistry
10 8
Bond order 1
2
79. Among the following the extensive variables are
(A) H (Enthalpy) (B) P (Pressure) (C) E (Internal energy) (D) V (Volume)
Ans : (A, C, D)
Hint : ‘P’ is an intensive variable
80. White phosphorus P4 has the following characteristics:
(A) 6 P – P single bonds (B) 4 P – P single bonds
(C) 4 lone pair of electrons (D) P – P – P angle of 60°
Ans : (A, C, D)
Hint : P P