[H_{3}O^{+}] = 0.00770 M
The equilibrium equation representing the dissociation of 

Given [H_{3}O^{+}] = 0.00770 M
Let the initial concentration of acid be x and change y
So y =
=
= 0.00770 M



0.00257 x - 0.00001979 = 0.00005929
x = 0.031 M
Therefore, initial concentration of the weak acid is <u>0.031 M</u>
<u>Answer:</u> The wavelength of the photon is 486.2 nm and it lies in the visible region
<u>Explanation:</u>
To calculate the wavelength of light, we use Rydberg's Equation:

Where,
= Wavelength of radiation
= Rydberg's Constant = 
= Higher energy level = 4
= Lower energy level = 2
Putting the values in above equation, we get:

Converting this into nanometers, we use the conversion factor:

So, 
As, the range of wavelength of visible light is 400 nm - 700 nm. So, the wavelength of the given photon lies in the visible region
Hence, the wavelength of the photon is 486.2 nm and it lies in the visible region