devarshi-dt-logo

Question:

The energy required to break one mole Cl−Cl bonds in Cl₂ is 242 KJ mol⁻¹. The longest wavelength of light capable of breaking a single Cl−Cl bond is:

640 nm

494 nm

594 nm

700 nm

Solution:

Energy required for one Cl₂ molecule = 242 × 10³ / NA Joules.
This energy is contained in the photon of wavelength λ.
hc/λ = E
= 6.626 × 10⁻³⁴ × 3 × 10⁸
λ = 242 × 10³ / 6.022 × 10²³
λ = 4947 Å ≈ 494 nm