Hydrogen iodide is placed in a reaction vessel from which the air has been removed. The vessel is then sealed and heated. Decomposition takes place, producing hydrogen and iodine. A quantity of hydrogen iodide is still present. Once equilibrium has been reached, the total pressure is $1.20 \times 10^5\,\text{Pa}$. The partial pressure of hydrogen is $4.00 \times 10^3\,\text{Pa}$. Under the conditions used, hydrogen iodide, hydrogen and iodine are all in the gaseous state. Calculate the value of $K_p$ for the equilibrium at this temperature, assuming the decomposition is the forward reaction.
- A$1.19 \times 10^{-3}$
- B$1.28 \times 10^{-3}$
- C$1.38 \times 10^{-3}$
- D$1.43 \times 10^{-3}$