Converting Between Kc and Kp

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

Kp = Kc(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 Kc to Kp
• 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 Kp 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 Kp.
Kp = (6.96 x 10-5)[(0.0821)(333)]2 = 0.052

Note: because we do not choose to use units for Kc and Kp , 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 Kp to Kc

• 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 Kc 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 Kc.
2.40  = Kc[(0.0821)(373)]-1
Kc = 73.5

Note: because we do not choose to use units for Kc and Kp , 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.