The standard molar enthalpy of formation (L) of 16.H2O only needs to be generated 16. H2O (L of water, so you need to divide the reaction enthalpy by 2 to get -286kJ/mol (sign, no need to change, because the enthalpy of formation is also enthalpy change) 17. First, the concentration of CD (dilute solution of acid, diluted H+) can be excluded. Then dilute it to the original 1/4. Because HCN is a weak acid, the concentration of H+ will not decrease so much. You can choose A from the perspective of multiple-choice questions. More strictly speaking, for the dilute solution of monobasic weak acid, c(H+)= radical sign (Ka×c) can be obtained from the ionization equilibrium equation, where c is the concentration of acid solute. (In the derivation of this formula, it is considered that the concentrations of H+ and acid radical ions are approximately equal. Therefore, when c is 1/4, c(H+) is 1/2.