[PJB]

Hello again,
    Here is another little problem that I can’t seem to figure out:

“Ammonium iodide dissociates reversibly to ammonia and hydrogen iodide if the compound is heated to a sufficiently high temperature:
                  NH4I(s) = NH3(g) + HI(g)
Some ammonium iodide is placed in a flask, and then heated to 400C.  If the total pressure in the flask when equilibrium has been achieved is 705 mm Hg, what is the value of Kp (when partial pressures are in atmospheres?)”  

What I did first was to the convert 705/760 mg Hg to .93 atm. Then I ignored the solid and multiplied .93^2(.93).  This is obviously not what I should have done because the back of the book says that the answer is Kp = 0.215

Any help would be very much appreciated – Thanks!!
Pam

[StudyGuide]

They didn't give you kc?

[Donaldson Tan]

total pressure = 705mmHg = 0.93atm

NH4I (s) <-> NH3 (g) + HI (g)

given there is zero ammonia and HI at the start, then amount of NH3 at equilibrium = amount of HI at equilibria, therefore

partial pressure of NH3 = partial pressure of HI = 0.93/2 = 0.465 atm

Kp = P_NH3 x P_HI = 0.465² = 0.216 atm²