Which among the following aqueous salt solution is used in conductivity cell to determine cell constant?

Calculate heat of formation of $$\mathrm{HCl}$$ gas from following reaction.

$$\mathrm{H}_{2(\mathrm{~g})}+\mathrm{Cl}_{2(\mathrm{~g})} \rightarrow 2 \mathrm{HCl}_{(\mathrm{g})} ; \Delta \mathrm{H}=-194 \mathrm{~kJ}$$

What is the value of frequency of radiation when transition occurs between two stationary states that differ in energy by $$\Delta \mathrm{E}$$ ?

What is molar concentration of weak monobasic acid if dissociation constant is 5 $$\times$$ 10$$^{-8}$$ and undergoes 0.5% dissociation?