I know my initial pressure of iodine is 1.00.

There's no mention of atomic iodine.

So I'm going to assume that the pressure of that is 0.

Now for the change row,

I have to use my coefficients from my balanced chemical equation, and

in doing so, I'll see I'm going to lose some of the I2 and I'm going to gain I.

But I'm going to gain 2 times the amount of I because of the 2 in my balanced

chemical equation.

For my equilibrium, I simply add up the initial and

change rows, so I have 1.00- x and 2x.

So now I have my equilibrium values with respect to x,

my equilibrium pressures with respect to x.

Now I can set up my KP expression,

so I know that KP = PI,

elemental iodine, squared / PI2.

Now I can substitute in the values that I know.

I have 2.91 x 10 to the -4th for my KP value.

That's going to be equal to the pressure of iodine, which note that is 2x.

I still have to square that because the pressure is 2x.

The law of mass action tells me that whatever that term is,

it must also be squared.

Note that the squared does go outside the parenthesis, so

that we square both the 2 and the x.

That's a common mistake that students make.

And on the bottom, we have the pressure by 2, which is 1.00- x.

And we're going to, again, make our simplifying assumption that x is much,

much less than 1.00.

And when we do, we can write out 2.91

x 10 to the -4th = 4x squared.