Equilibrium constant expression

Hello
I’m solving previous exams and there is this exercise that appears in almost every exam :
1. The problem statement, all variables and given/known data

Suppose v moles of a gas H2O are enclosed in a constant volume V at a low temperature so the gas is in aN undissociated state. At highest temperatureS, H2O molecules are dissociated
2H2O-> 2H2 + O2

If "f" is the fraction of H2O molecules that are dissociated at a known temperature T, and p the total pressure of the gas at that temperature, show that Kp(T) at this pressure is :
Kp(T)= p f^3/ (( 1-f)^2(2+f) )

2. Relevant equations

3. The attempt at a solution
Kp(T)= (PH20/p)²/ [ (P02/p)*(PH2/p)² ]
= p*PH20²/ [ P02*PH2² ]

And then I dont know how to get to the given expression :confused:

http://ift.tt/1jG2X0I

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