A gaseous mixture of propane, acetylene and CO 2 is burnt in excess of air. Total 4800 kJ heat is evolved. The total volume of CO 2 (g) after combustion is 224 liters at NTP.
The total evolved heat is used to perform two separate process :
(i) Vapourising 87.5% of water (liquid) obtained in the process of burning the original mixture.
(ii) Forming 3808 liters ethylene measured at STP from its elements.
Δ H H–H = 435 kJ/mol Δ H C–H = 416 kJ/mol Δ H C–C = 347 kJ/mol
Δ H C=C = 615 kJ/mol, Δ H C ≡ C = 812 kJ/mol Δ H sublimation of (C,s) = 718 kJ/mol
(CO 2 , g) = –394 kJ/mol 
(H 2 O, λ ) = – 286 kJ/mol. 
(H 2 O, g) = – 246 kJ/mol.
(i) 
for C 2 H 2 (g) + H 2 (g) ⎯→ C 2 H 4 (g)
Text Solution
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(i)d (ii)b (iii)a
Let moles of C 3 H 8 = x, moles of C 2 H 2 = y & moles of CO 2 = z.
Calculation of 
of C 3 H 8 (g)
3C(s) + 4H 2 . (g) ⎯→ C 3 H 8 (g)
of C 3 H 8 (g) = [3(718) + 4(435)] – [2(347) + 8 (416)]
= 3894 – 4022 = – 128 kJ / mol.
Calculations of 
of C 2 H 2. (g)
2C (s) + H 2 (g) ⎯→ C 2 H 2 (g)
of C 2 H 2 (g) = [2(718) + (435)] – [(812) + 2(416)]
= (1436 + 435) – [1644]
= 227 kJ/mol.
Calculations of 
of C 2 H 4 (g)
2C (s) + 2H 2 (g) ⎯→ C 2 H 4 (g)
of C 2 H 4 (g) = 2 (718) + 2 (435) – [615 + 4(416)]
= 2306 – 2279 = 27 kJ/mol.
Calculation of 
of C 3 H 8 (g)
C 3 H 8 (g) + 5O 2 (g) ⎯→ 3CO 2 (g) + 4H 2 O ( λ )
of C 3 H 8 = [3 
(CO 2 ) + 4 
(H 2 O, λ )] – 
(C 3 H 8 , g)
= [3 (–394) + 4 (–286)] – (–128) = –2198 kJ/mol.
Calculation of 
of C 2 H 2 (g)
C 2 H 2 (g) + 2.5O 2 (g) ⎯→ 2CO 2 (g) + H 2 O ( λ )
of C 2 H 2 = [2 
(CO 2 ) + 
(H 2 O, λ )] – 
(C . H . )
= [2 (–394) + (–286)] – 227 = –1301 kJ/mol.
Now total heat released during combustion
2198 x + 1301 y = 4800 (i)
Combustion = 3x + 2y + z = 10 (ii)
Total moles of H . O ( λ ) formed = 4x + y.
moles of C . H . (g) to be prepared = 
= 170.
Total heat absorbed during evaporation of water formation of 170 moles C . H . = 4800.
[(4x + y) × 0.875 x 40] + (170 x 27) = 4800
4x + y = 6 (iii)
on solvent (i), (ii) and (iii)
we get x = 1, y = 2, and z = 3
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