# What is the relationship between Kc and Kp?

1. ### dnt

239
what is the relationship between the equilibrium constant Kc and the equilibrium pressure constant Kp? I am given many questions that either give Kc and ask for partial pressures of gases or vice versa. what is the best way to approach these questions?

another question states given this equation:

SO2 + NO2 --> SO3 + NO

Kc=2.9

how much NO2 must be added to 2.2 mol of SO2 to make 1.3 mol of SO3 and equilibrium? I just dont know how to start this. any help would be appreciated. thanks.

2. ### aniketp

79
kp=kc*RT$$\Delta$$n ]$$\Delta$$n=total change in the no. of moles

3. ### dnt

239
what about the 2nd question? what i think needs to be done is to assume the number of moles for SO3 and NO is the same because both products are made in equal quantities at equilbrium - is that the correct assumption?

then i can plug all the numbers into the Kc equation and solve from there?

4. ### dnt

239
ok well how do i get Kp from Kc if no temperature is given?

A <--> 2B

initial partial pressure of A is 0 and initial partial pressure of B is 1 atm. they ask to find the equilibrium partial pressures of A and B and they give Kc value.

im pretty sure this is an ICE problem

for A: I=0, C=x, E=x
for B: I=2, C=-x, E=2-x

but my problem is i cannot find Kp to set up the equation (x)/(2-x)^2 = Kp...i am only given Kc=1.4

(is it safe to assume the reaction is going to the left, towards A, because the initial partial pressure of A is 0 therefore there isnt any A to start?)

5. ### dnt

239
bump:

found one correction i think:
for A: I=0, C=x, E=x
for B: I=2, C=-2x, E=2-2x

but still cannot connect these values (which are pressures) to a given Kc value without temperature. what should i try next?

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