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Thermodynamics

# Given that $C+O_2 \rightarrow CO_2;\: \: \: \: \: \Delta H^{\circ} = -x\: kJ\: and\: 2CO+O_2 \rightarrow 2CO_2; \: \: \: \: \:\Delta H^{\circ} = -y\: kJ$  The standard enthalpy of formation of carbon monoxide is

$\begin {array} {1 1} (A)\;y-2x & \quad (B)\;\large\frac{(2x-y)}{2} \\ (C)\;\large\frac{(y-2x)}{2} & \quad (D)\;2x-y \end {array}$

Multiplying eq. (ii) by $\large\frac{1}{2}$ and subtracting it from eq. (i) to get
$C(s) +\large\frac{1}{2}$$O_2 \rightarrow CO;\: \: \: \: \: \: \: \: \: \: \Delta H=\large\frac{(y-2x)}{2}$
Ans : (C)