Converting Between K_c and K_p

To convert between K_c to K_p use the following equation which is based on the relationship between molarities and gas pressures.

K_p = K_c(RT)^Dn

Dn is the difference in the number of moles of gases on each side of the balanced equation for the reaction.

Dn = (number of moles of gaseous products - number of moles of gaseous reactants)

Converting K_c to K_p
Converting K_p to K_c

Converting K_c to K_p

Calculate the difference in the number of moles of gases.
Subsititute Dn, R, and T into the equation and solve.

Example: Calculate the value of K_p for the following reaction, at 333 K.

Calculate the difference in the number of moles of gases, Dn.

Dn = (2 moles of gaseous products - 0 moles of gaseous reactants) = 2

Substitute the values into the equation and calculate K_p.

K_p = (6.96 x 10^-5)[(0.0821)(333)]² = 0.052

Note: because we do not choose to use units for K_c and K_p , we cannot cancel units for R and T. However, be careful to use the value of R consistent with the units of pressure used in the problem, and T in Kelvin.

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Converting K_p to K_c

Calculate the change in the number of moles of gases.
Subsititute Dn, R, and T into the equation and solve.

Example: Calculate the value of K_c at 373 K for the following reaction:

Calculate the change in the number of moles of gases, Dn.

Dn = (2 moles of gaseous products - 3 moles of gaseous reactants) = - 1

Substitute the values into the equation and calculate K_c.

2.40 = K_c[(0.0821)(373)]^-1
K_c = 73.5

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Converting Between Kc and Kp

Converting Between K_c and K_p