Question

The energy required to break one mole of $Cl-Cl$ bonds in ${Cl}_2$ is $242$ $kJ$ ${mol}^{-1}$. The longest wavelength of light capable of breaking single $Cl-Cl$ bond is:
$(c=3\times {10}^{8}m{sec}^{-1},N_A=6.023\times {10}^{23}{mol}^{-1})$

• A. $700nm$
• B. $494nm$
• C. $594nm$
• D. $640nm$

Answer 1

The correct option is B $494nm$

For calculation of longest wavelength of light that can break Cl-Cl bond can be found by using Formula E = Nhc/$\lambda$

where h= planck's constant, c = speed of light, $\lambda$ = wavelength of light, n = no of molecules (no. of photon required) and $N_A$= Avogadro's Number

$\lambda$ = $\frac{6.62\times {10}^{-34}\times 3\times {10}^8\times N_A}{242\times {10}^3}$ = 492 nm

so , answer is B.