Identify the law for the statement "Overall the enthalpy change for a reaction is equal to sum of enthalpy changes of individual steps in the reaction"

Calculate $\triangle G^\circ$ for reaction,
$\text{CH}_{4(g)} + \text{H}_{2(g)} \rightarrow \text{C}_2\text{H}_{6(g)}$ ($K_p = 2 \times 10^{17}$, $R = 8.314\ \text{JK}^{-1}\text{mol}^{-1}$)

What is the normal pH range of human blood?

Calculate the pOH of $0.01$ M monobasic acid that is completely dissociated, at $298$ K.