[CIE alevel]

Hi,
**Question:
In an experiment , b mole of hydrogen iodide were put into a sealed vessel under pressure p . At equilibrium, x mole of the hydrogen iodide had dissociated , the reaction being represented by the following equation ,

2HI(g) <---> H2(g) + I2(g)

Express Kp in terms of b and x .

**I know,


Will there be p, pressure (1 of terms from the question) in the above Kp equation?
If so then, don't we express Kp in terms of b, x and p?
What is done with "sealed vessel under pressure p"??

What do i equate :

Let me know asap. Thanks

[typhoon2028]

You need to convert to partial pressures, and also mole fractions.

In this problem you start with total moles of HI = b
x amount of HI reacts to H2 & I2.

Resultant moles of HI = (b-x)
x = moles of H2 + moles of I2
moles of I2 = moles H2
H2 moles = x/2
I2 moles = x/2

Total moles never changes, so total moles = b.

Kp = P_H2 * P_I2 / (P_HI)²

P_H2 = partial pressure of H2, which equals X_H2 * P_total

X_H2 = mole fraction of H2, which equals xb/2

If you do all the algebra you will see the Ptotal terms will cancel out and you are left with an equation in terms of x and b.