Q1(p). Energy of one mole of 2500 Å photons

Answer in brief

4.785 × 10¹² erg mol⁻¹ = 1.144 × 10⁵ cal mol⁻¹ = 2.99 × 10²⁴ eV mol⁻¹ (4.96 eV per photon).

Key points

• One photon: E = hc/λ = (6.626 × 10⁻³⁴)(2.998 × 10⁸)/(2.5 × 10⁻⁷) = 7.946 × 10⁻¹⁹ J.
• One mole (einstein): 7.946 × 10⁻¹⁹ × 6.022 × 10²³ = 4.785 × 10⁵ J mol⁻¹.
• 1 J = 10⁷ erg: E = 4.785 × 10¹² erg mol⁻¹; ÷ 4.184: E = 1.144 × 10⁵ cal mol⁻¹.
• Per photon 4.96 eV; × NA: E = 2.99 × 10²⁴ eV mol⁻¹.
• 478.5 kJ mol⁻¹ exceeds most bond energies: 250 nm UV can break bonds.