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Law of mass action Kc & ICE Table


I have a question about the following problem:

H2 + I2 ::equil:: 2HI
Calculate the percentage of HI at equilibrium, if HI = 1mol in the beginning and Kc = 25

I tried to solve this problem using the ICE Table:

c[H2] = 0 + x
c[I2] = 0 + x
c[2HI] = 1 - 2x

according to this x=1/3 (…)

However my teacher calculated something different:

c[H2] = 0.5x
c[I2] = 0.5x
c[2HI] = 1-x

-> x≈0.29

What did I do wrong here? Can I not use the ICE Table in this case? It usually works… (To be clear, my teacher does not use the ICE Table but she approved it, so I am allowed to use it. But because she doesn’t use it, her solutions are very confusing because she doesn’t show the intermediate steps.)

Thanks for any answers! (PS english is not my native language so I apologize for any mistakes, especially in the technical language)

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