Answer : The equilibrium concentration of
in the trial solution is 
Explanation :
First we have to calculate the initial moles of
and
.

and,

The given balanced chemical reaction is,

Since 1 mole of
reacts with 1 mole of
to give 1 mole of 
The limiting reagent is, 
So, the number of moles of
= 0.0020 mmole
Now we have to calculate the concentration of
.

Using Beer-Lambert's law :
where,
A = absorbance of solution
C = concentration of solution
l = path length
= molar absorptivity coefficient
and l are same for stock solution and dilute solution. So,

For trial solution:
The equilibrium concentration of
is,
![[SCN^-]_{eqm}=[SCN^-]_{initial}-[FeSCN^{2+}]](https://tex.z-dn.net/?f=%5BSCN%5E-%5D_%7Beqm%7D%3D%5BSCN%5E-%5D_%7Binitial%7D-%5BFeSCN%5E%7B2%2B%7D%5D)
= 0.00050 M
Now calculate the
.

Now calculate the concentration of
.
![[SCN^-]_{eqm}=[SCN^-]_{initial}-[FeSCN^{2+}]](https://tex.z-dn.net/?f=%5BSCN%5E-%5D_%7Beqm%7D%3D%5BSCN%5E-%5D_%7Binitial%7D-%5BFeSCN%5E%7B2%2B%7D%5D)
![[SCN^-]_{eqm}=(0.00050M)-(9.17\times 10^{-5}M)](https://tex.z-dn.net/?f=%5BSCN%5E-%5D_%7Beqm%7D%3D%280.00050M%29-%289.17%5Ctimes%2010%5E%7B-5%7DM%29)
![[SCN^-]_{eqm}=4.58\times 10^{-8}M](https://tex.z-dn.net/?f=%5BSCN%5E-%5D_%7Beqm%7D%3D4.58%5Ctimes%2010%5E%7B-8%7DM)
Therefore, the equilibrium concentration of
in the trial solution is 