Kc is the equilibrium constant with respect to concentration and Kp is the equilibrium constant with respect to pressures. When the moles of gas on both sides of the equation are the same, these two values are equal. When there are different numbers of moles of gas on either side,...
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Kc is the equilibrium constant with respect to concentration and Kp is the equilibrium constant with respect to pressures. When the moles of gas on both sides of the equation are the same, these two values are equal. When there are different numbers of moles of gas on either side, as in this reaction, then we have to use the following relationship
Kp = Kc(RT)^deltan
Where R is the ideal gas constant, T is the temperature in Kelvin (assuming T = 25 degC since no temperature is given), and deltan is the change in number of moles (products - reactants).
Since we know Kp and are trying to find Kc, we can rearrange the equation to
Kc = Kp/((RT)^deltan)
Kc = (1.5x 10^-5)/((0.08206*298)^(2-4))
Kc = (1.5x10^-5) / 0.00167
Kc = 8.97 x 10^-3