When 1.00 g of gaseous I2 is heated to 1000. K in a 1.00 L sealed container, the resulting equilibrium mixture contains 0.830 g of I2. Calculate Kc for the dissociation equilibrium below.

I2 <--> 2 I

What I did was converted the initial I2 into moles then molarity since it is just over 1 L (.00394 M) then I found the molarity of I by subtracting .830 from 1 and converting first to moles (.00134 M of I) I then set up Kc= [.00134 M of I]^2/ [.00394 M of I2] which i found to be .00045...but it's incorrect. I'm not sure what I am missing...

Your procedure appears to be ok down to the last step. The divisor should be the 0.830/253.8 = 0.00327 and not 0.00395 since the 0.830 g was the equilibrium amount. Also, watch the number of significant figures. That sometimes causes problems.

Realize there are 0.830 g of I2 at equilibrium... not I

To calculate the equilibrium constant, Kc, for the dissociation equilibrium of I2 <--> 2I, you correctly started by converting the initial amount of I2 into moles and then molarity. However, there seems to be an error in your calculation. Let me guide you through the correct steps:

Step 1: Convert the mass of I2 to moles:
Given mass of I2 = 1.00 g
Molar mass of I2 = 253.8 g/mol
Moles of I2 = (1.00 g) / (253.8 g/mol) = 0.00394 mol

Step 2: Calculate the initial molarity of I2:
Volume of container (V) = 1.00 L
Initial molarity of I2 (M) = moles / volume
Molarity of I2 = 0.00394 mol / 1.00 L = 0.00394 M

Step 3: Calculate the final molarity of I:
To find the final molarity of I, you correctly subtracted the mass of I2 formed from the initial moles of I2. However, instead of converting to moles first, we should directly convert the mass of I2 formed to molarity using the volume of the container.

Mass of I2 formed = 0.830 g
Final molarity of I = (0.830 g) / (253.8 g/mol) / (1.00 L) = 0.00326 M

Note that we used the molar mass of I2 and the volume of the container to directly convert mass to molarity.

Step 4: Apply the formula for Kc:
The equilibrium constant expression for the dissociation of I2 is:

Kc = [I]^2 / [I2]

Substituting the calculated values:
Kc = (0.00326 M)^2 / (0.00394 M) = 0.00270

Therefore, the correct value for Kc is 0.00270.