Answer
288 kJ/mol
Work Step by Step
Because the solution is yellow, we can deduce that purple light is absorbed since purple is the complementary color to yellow.
The color purple ranges from 400 to 430 nm, so we can estimate the average wavelength as 415 nm.
Energy corresponding to this wavelength is equal to the crystal field splitting energy $\Delta$.
That is,
$\Delta= \frac{hc}{\lambda}=\frac{(6.626\times10^{-34}\,J\cdot s) (3.00\times10^{8}\,m/s)}{415\times10^{-9}\,m}$
$=4.78988\times10^{-19}\, J$
In kJ/mol,
$\Delta=4.78988\times10^{-19}\, J\times\frac{kilo}{1000}\times\frac{6.02\times10^{23}}{mol}$
$=288\, kJ/mol$
