[cokuardi]

Hi, i need help in solving this problem:
 
The equilibrium constant Kc for the dissociation of iodine in the gas phase,
I2(g) « 2 I(g)  
 
is 1.70E+2 at 1000 K. Suppose that 5.00 mol of I2 gas is initially present in a 2.60-L flask at 1000 K. A student measures the equilibrium concentration of the iodine atoms (not the molecules) in the flask and obtains a value of 4.61E-1. On hearing this result, the professor calculates the reaction quotient, scoffs at the purported measurement, and sends the student back to the lab. What is the reaction quotient for this mixture?

[cokuardi]

I've tried multiple variations, but I can't seem to get the right answer.  I can't seem to find the initial [] of 2I.

[Borek]

What do you mean by 'initial'? Initial is probably 0.

Write down equation for reaction quotient.

[cokuardi]

its not 0, Q=²/(I₂)

[Borek]

its not 0

Define initial. If by initial you mean 'concentration before reaction started' - it is 0. If you mean something else - it is not initial.

Q=²/(I₂)

OK. So you need two concentrations (or partial pressures) - I₂ and I. Now write down mass balance.

[cokuardi]

I don't know what u mean by that.

[Pelle]

Take a piece of paper and do a table:


Reaction:   I2 -> 2 I

Before eq.: 5        0

At eq.:


Figure out what the values should be at eq. and put them into equilibrium equation. Solve the equation.

If you get stuck then go ahead and ask again.

[cokuardi]

I'm not trying to find the equilibrium concentrations, and I already have the equilibrium concentration for I.  I need the concentration of it initially.

[Borek]

I don't know what u mean by that.

Have you heard about mass conservation law? Total mass of iodine is not changing - so for every I₂ dissociated two I appears. Try to express it using a formula. ICE table proposed by Pelle is based on the same law, just applied in slightly different form.

[cokuardi]

   I₂<------------->2I
I  1.923 M             ?
C   -x                +2x
E  1.923-x       2x=4.61x10^-1

x=2.3x10^-1

so

   I₂<------------->2I
I  1.923 M             ?
C   -2.3x10^-1      4.61x10^-1
E  1.69 M             4.61x10^-1

[Borek]

I need the concentration of it initially.

I think the question is badly worded. It is not obvious for me whether there is 5.0 mole of iodine plus free I, or whether the experiment started with 5.0 moles of I₂. Is that your problem?

My first understanding was that there is total of 1269 g of iodine in both diatomic and dissociated form (mass of 5.0 moles of I₂) - in other words experiment started with 5.0 moles of I₂ and initial amount of I was zero. That's how these questions are usually constructed.

[cokuardi]

It is badly worded, but since i'm trying to find Q, it is the initial amount of I and I₂.  I think the 5 moles is just for I₂.  I am quite confussed on this problem

[cokuardi]

This is what the website that i input the answers into said:  Q = ^2 / [I2] .. substitute the values, use the final I_2 concentration Remember that there is a mistake in the measurement!

[cokuardi]

Someone PLEASE *delete me*!!!

[Borek]

Do as I told you - assume initial concentration of I is 0.