How Is the Equilibrium Constant Calculated for I2 and I- in KI Solution?

[zorro]

Homework Statement

A saturated solution of iodine in water contains 0.330g of I2/L. More than this can dissolve in a KI solution because of the following equilibrium-

I2(aq) +I-(aq) ------> I3- (aq) where 2 and 3 are subscripts and - is negative charge

A 0.1M KI solution actually dissolves 12.5g of iodine/L, mmost of which is converted to I3-. Assuming that the concentration of I2 in all saturated solutions is same, calculate the equilibrium constant for the above reaction.

Homework Equations

The Attempt at a Solution

0.33g/L is 0.0013 mole and 12.5g/L is 0.0492 mole
I don't know how to proceed

 

[Borek]

Start writing expression for equilibrium constant. Then think which concentrations you are given, and which you can calculate from the simple stoichiometry.

 

[zorro]

K=[I3-]/[I2][I-]

Actually, I did not understand the problem. A saturated solution of Iodine in water is given to us.
What is the meaning of 'A 0.1M KI solution actually dissolves 12.5g of iodine/L'
how does this info help us?

 

[Borek]

Why does the iodides solution contain more iodine than solution in pure water?

 

[zorro]

It is so because KI is added to it.
0.1M KI dissolves 0.0492 mole I2
but we have only 0.0013 mole I2 so all of it dissolves and converts into I3-
moles of KI left = 0.1-0.0013
Is this approach correct?

 

[Borek]

No, there was excess iodine present. It was not dissolved, but it was added as a solid, thus there is much more I₃^- present.

How much? That's where the mass of dissolved iodine comes handy.

 

[zorro]

excess iodine present in KI solution or saturated solution of Iodine in water?

 

[Borek]

No idea what you are asking about. SOLID iodine. Solutions are in contact with the solid. Concentrations given are for the SOLUTIONS.

 

[zorro]

I would be thankful to you if you write the concentrations at equilibrium as this question is very vague to me.

 

[Borek]

Concentration of free iodine is identical with the concentration in pure water. Total concentration of iodine dissolved is sum of free iodine and I₃^-. Simple subtraction will give you concentration of I₃^-, then simple stoichiometry will tell you how much I^- was left.

 

[zorro]

Thanks a lot. I understood it clearly now.