Question

Sodium atoms emit light with a wavelength of 330 nm when an electron moves from a 4p orbital to a 3s orbital.

Answer 1

E= hv

Where E= energy (here the difference in energy)

h = plancks constant

v= frequency of light

v= c/lamda

There fore

E= hc/lamda

C= velocity of light = 2.99 x 10⁸ m/s

Lamda = wavelength

Here

h= 6.626 x 10^-34 joules

c=2.99 x 10⁸ m/s

lamda = 330 nm = 3.30 x 10^-7 m

E = (6.626 x 10^-34 joules) x (2.99 x 10⁸ m/s) /3.30 x 10^-7 m

= 6.00 x 10^-22 kJ (per photon)

The number of molecules per mole is known as Avogadro's constant, and is 6.022×10²³ mol⁻¹

For a mole of photons:
E = 6.00 x 10-22 kJ/photon x 6.022 x 10²³ photons/mol
= 361 kJ/mol