A sample of CaSO4(s) is placed in a container and then heated so that it decomposes according to the following equation:

A sample of CaSO4(s) is placed in a container and then heated so that it decomposes according to the following equation:

2 CaSO4(s) ==> 2 CaO(s) + 2 SO2(g) + 02(g) At equilibrium, the total pressure in the container was found to be 0.60 atm at 700 C. Calculate Kp-

A) 0.016

B) 0.20

C) 0.40

D) 0.032

E) 0.0080

Answer

The reaction is as:

• CaSO4(s) --> 2CaO(s) + 2SO2(g) + O2(g)

Total pressure = 0.60atm at 700oC.

Kp = ?

We know, equilibrium Kp is written via the formula

• Kp = Product of Partial pressure of products/ Product of Partial pressure of reactants = (pSO2(g))^2 x pO2(g) 
• where pSO2(g) and pO2(g) are the partial pressures of the SO2 and O2 gas 

Furthermore, at equilibrium; Total pressure is the sum of the partial pressures of SO2 and O2 gas.

• Total pressure, pTOTAL = pSO2(g) + pO2(g) 

At equilibrium, the pressures of the gases become:

pSO2(g) = 2x

pO2(g) = x

Meaning, 2x + x = pTOTAL

2x + x = 0.60atm

3x = 0.60atm

x = 0.60atm/3 = 0.2atm

pSO2(g) = 2x = 2 x 0.2atm = 0.4atm 

pO2(g) = x = 0.2atm

Thus, the value of Kp is as:

Kp = (pSO2(g) )^2 x pO2(g) 

Kp = (0.4 atm)^2 x 0.2vatm = 0.032

Kp = 0.032

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