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Question

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

A
45.9
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B
47.8
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C
20.0
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D
47.3
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Solution

The correct option is B 20.0
First reaction is formation reaction of carbon dioxide. Hence, heat of formation of carbon dioxide = 94.2Kcal.
Second reaction is formation reaction of water. Hence, heat of formation of water = 68.3kCal.
Since, ΔH for third reaction = 210.8Kcal
Also, ΔH = ΔHf(CO2)+2×ΔHf(H2O)ΔHf(CH4)
=> 210.8=94.2+2×68.3ΔHf(CH4)
=> ΔHf(CH4)=94.2+136.6210.8=20Kcal.
Hence, answer is option C.

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