# Homework Help: Tough Problem on Chemical Equilibrium

1. Sep 17, 2010

### zorro

1. The problem statement, all variables and given/known data

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.

2. Relevant equations

3. The attempt at a solution

0.33g/L is 0.0013 mole and 12.5g/L is 0.0492 mole
I dont know how to proceed

Last edited: Sep 17, 2010
2. Sep 17, 2010

### Staff: Mentor

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

Last edited by a moderator: Aug 13, 2013
3. Sep 17, 2010

### 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?

4. Sep 17, 2010

### Staff: Mentor

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

5. Sep 17, 2010

### 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?

6. Sep 17, 2010

### Staff: Mentor

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.

7. Sep 18, 2010

### zorro

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

8. Sep 18, 2010

### Staff: Mentor

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

9. Sep 18, 2010

### zorro

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

10. Sep 18, 2010

### Staff: Mentor

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.

Last edited by a moderator: Aug 13, 2013
11. Sep 18, 2010

### zorro

Thanks alot. I understood it clearly now.