True!! they can be measured without changing chemical identity.
<span>The velocity of a wave is calculated by v = λf. From this equation we can take the frequency f = v / λ and substitute in the energy equation to find a new equation for the calculation of the energy from the wavelength:
E = hc / λ</span>
To calculate the photon energy, in eV (Electrovolt), when we have the value of the wavelength λ (necessarily given in Å - angstrom), we use an alternative equation given by:

Data: 670.8 Å
Solving:


2H₂ + O₂ → 2H₂O
mole ratio of O₂ : H₂O is 1 : 2
∴ if moles of O₂ = 16 mol
then moles of H₂O = (16 mol × 2)
= 32 mol
Actual question from source:-
A 3.96x10-4 M solution of compound A exhibited an absorbance of 0.624 at 238 nm in a 1.000 cm cuvette. A blank had an absorbance of 0.029. The absorbance of an unknown solution of compound A was 0.375. Find the concentration of A in the unknown.
Answer:
Molar absorptivity of compound A = 
Explanation:
According to the Lambert's Beer law:-
Where, A is the absorbance
l is the path length
is the molar absorptivity
c is the concentration.
Given that:-
c = 
Path length = 1.000 cm
Absorbance observed = 0.624
Absorbance blank = 0.029
A = 0.624 - 0.029 = 0.595
So, applying the values in the Lambert Beer's law as shown below:-

<u>Molar absorptivity of compound A =
</u>
50/35 = √x/ √31.9988 g
50 x (√31.9988/ 35) = √x
√x = 8.081...
x= 65.30...
x= 70 g/ mol