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}

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
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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