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 a single $Cl - Cl$ bond is
($c = 3 \times 108 \,ms^{-1}$ and $NA = 6.02 \times 10^{23}\, mol^{-1}$)

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

Answer 1

Answer: (D)

Energy required for $1\, Cl_2$ molecule $= \frac{242\times10^{3}}{N_{A}}$ Joules.
This energy is contained in photon of wavelength $‘\lambda’$.
$\frac{hc}{\lambda} = E \Rightarrow \frac{6.626\times 10^{-34}\times 3\times 10^{8}}{\lambda} = \frac{242\times 10^{3}}{6.022\times 10^{23}}$
$\lambda = 4947\,\mathring{A} \approx 494\, nm$