Question

If light has a wavelength of 695 nm, what is the energy of this light expressed with units of kJ/mol?

Answer 1

Ans: 172.26 kJ/mol

According to Planck - Einstein's equation, E = hv

where, E = Energy of light

h = Planck's constant, equal to 6.626x10⁻³⁴ J s

v = frequency of light

Now,  E = hν

Also, ν.λ = c where λ = wavwlenght of light

c = speed of light, equals to 3x10⁸ m s⁻¹

Hence, E = h.c / λ

So, E = 6.626x10⁻³⁴ J s X 3x10⁸ m s⁻¹ / 695 x 10^-9 m

E = 19.878 x 10^-26 J m/ 695 x 10^-9 m

E = 0.0286 x 10^-17 J

E = 2.86 x 10^-22 kJ

Now, for kJ/mol, multiply E by Avogadro no. i.e. 6.023 x 10²³

E (kJ/mol) = 2.86 x 10^-22 kJ x 6.023 x 10²³ kJ/mol

E = 17.226 x 10 kJ/mol

E = 172.26 kJ/mol