Answer:
The value of 
for this reaction at 1200 K is 4.066.
Explanation:
Partial pressure of water vapor at equilibrium = 
Partial pressure of hydrogen gas at equilibrium = 
Total pressure of the system at equilibrium P = 36.3 Torr
Applying Dalton's law of partial pressure to determine the partial pressure of hydrogen gas at equilibrium:
The expression of is given by:
The value of for this reaction at 1200 K is 4.066.
We assume that we have Ka= 4.2x10^-13 (missing in the question)
and when we have this equation:
H2PO4 (-) → H+ + HPO4-
and form the Ka equation we can get [H+]:
Ka= [H+] [HPO4-] / [H2PO4] and we have Ka= 4.2x10^-13 & [H2PO4-] = 0.55m
by substitution:
4.2x10^-13 = (z)(z)/ 0.55
z^2 = 2.31x 10^-13
z= 4.81x10^-7
∴[H+] = 4.81x10^-7
when PH equation is:
PH= -㏒[H+]
= -㏒(4.81x10^-7) = 6.32
0.500 moles is roughly .5*6.022*10^23=3.011*10^23 atoms. This is independent of STP.
