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Question

The heat liberated on complete combustion of 7.8g benzene is 327 KJ. This heat has been measured at constant volume and at 270 C. What is the heat of combustion of benzene at constant pressure at 270 C (R = 8.3 J mol1 K1)


A

-3273.735 KJ

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B

-3274.735 KJ

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C

-3275.735 KJ

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D

-3270.735 KJ

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Solution

The correct option is A

-3273.735 KJ


Let us first write the chemical reaction which is

C6H6(l)+152O2(g)6CO2(g)+3H2O(l)

Δng = number of moles of gaseous products - number of moles of gaseous reactants

Δng = 6152 = 32

Now ΔU per mole = 327×787.8=3270KJ

ΔH=ΔU+ΔnRT=3270+(32)(8.3)(300)103

= 3270+(32)(8.3)(300)103

ΔH=3273.735KJ


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