b
(b) \(C + {O_2} \to C{O_2}\,;\,\,\,\Delta {H_F} = - 490\,KJ/mol\) \(-(I)\)
\({H_2} + 1/2{O_2} \to {H_2}O;\,\,\Delta {H_F} = - 240\,KJ/mol\) \(-(II)\)
\(8C + 18H \to {C_8}{H_{18}};\,\,\Delta {H_F} = + 160\,KJ/mol\) \(-(III)\)
applying
\((I)\) \( \times \,8 + \) \((II)\) \( \times 9 + \) \((III)\)
\({C_8}{H_{18}} + \frac{{25}}{2}{O_2} \to 8C{O_2} + 9{H_2}O\)
\(\Delta {H^o} = - 3920 - 2160 - 160 = 6240\,KJ/mol\)
\(\Delta {H^o} = \) for \(6\) moles of octane \( = 6240 \times 6\)
\( = 37440\,KJ/mol = - 37.4\,KJ\)