What is Hess’s law? Why is it useful?

Solution: Step1: a) The relation between wavelength() and frequency() of radiation is given by: = c / ------(1), where c is the velocity of light. In the above problem, given, wavelength of light = 532 nm = 532 x 10 m -9 [1 nm = 10 m]-9 = 5.32 x 10 m -7 8 Velocity of light, c = 3 x 10 m/sec Substituting the values in equation (1), we get, 8 -7 frequency() = ( 3 x 10 m/sec) / (5.32 x 10 m) 15 -1 = 0.564 x 10 sec . = 5.64 x 10 sec . -1 14 -1 Thus, frequency of the radiation is 5.64 x 10 sec . Step2: b) According to Planck’s quantum theory of radiation, the amount of energy associated with a quantum(in case of light, the quantum of energy is often called photon) of radiation is proportional to the frequency of light, E = h , where h is a universal constant called Planck’s constant. is the frequency and E is the energy of a photon. Or, E = hc/ -------(2), where, is the wavelength of light and c is the velocity. Given, wavelength of the light = 532 nm -9 -9 = 532 x 10 m [1 nm = 10 m] = 5.32 x 10 m -7 -34...