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Question

Given,
C(s)+O2(g)CO2(g)    ΔH=94.2 kcal
H2(g)+12O2(g)H2O(l)    ΔH=68.3 kcal
CH4(g)+2O2(g)CO2(g)+2H2O(l)   ΔH=210.8 kcal
The heat of formation of methane in kcal will be :


Your Answer
A
50 kcal
Your Answer
B
40 kcal
Your Answer
C
30 kcal
Correct Answer
D
20 kcal

Solution

The correct option is B 20 kcal
Given, C(s)+O2(g)CO2(g)    ΔH=94.2 kcal....(1)
H2(g)+12O2(g)H2O(l)    ΔH=68.3 kcal....(2)
CH4(g)+2O2(g)CO2(g)+2H2O(l)   ΔH=210.8 kcal....(3)
we need C(s)+2H2(g)CH4(g)

multiply equation 2 by 2, we get
2H2(g)+O2(g)2H2O(l)        ΔH=+2×(68.3) kcal............4

reversing the equation 3, we get 
CO2(g)+2H2O(l)CH4(g)+2O2(g)   ΔH=(+210.8) kcal.........5

adding  eqn 1, 4 and 5 to get  C(s)+2H2(g)CH4(g)
so, heat of formation of methane =(94.2+2×(68.3)+210.8) kcal
=20 kcal
 

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