go to the AUS-e-TUTE homepage

KP and Kc Tutorial

Key Concepts

Please do not block ads on this website.
No ads = no money for us = no free stuff for you!

Worked Example : Calculating Equilibrium Partial Pressure

Question: A container at 800°C was filled NOCl gas which decomposes to form NO gas and chlorine gas.
The equilibrium partial pressure of NOCl was 0.657 atm, what was the partial pressure of NO gas?
KP = 1.8 × 10-2 for the reaction NOCl(g) ⇋ NO(g) + ½Cl2(g)

Solution:
(Based on the StoPGoPS approach to problem solving in chemistry.)

  1. What is the question asking you to do?

    Calculate the partial pressure of NO gas.
    P(NO(g)) = ? atm

  2. What data (information) have you been given?

    (a) NOCl(g) ⇋ NO(g) + ½Cl2(g)
    (b) KP = 1.8 × 10-2
    (c) P(NOCl(g)) = 0.657 atm

  3. What is the relationship between what you know and what you need to find out?

    Write the expression for KP

    KP = P(NO)P½(Cl2)

    P(NOCl)

    Write expressions for the equilibrium partial pressure of each species based on the reaction stoichiometry (mole ratio):
    P(NOCl) = 0.657 atm
    P(NO) = x atm
    P(Cl2) = ½x atm

  4. Subsitute the values into the expression for KP and solve for P(NO):

    1.8 × 10-2 = xx)½

    0.657

    Simplify the expression for KP and solve for x
    1.8 × 10-2 × 0.657= (½x3)½
    0.0118 = (½x3)½
    (0.0118)2 = ½x3
    1.4 × 10-4 = ½x3
    1.4 × 10-4 × 2 = x3
    2.8 × 10-4 = x3
    3√2.8 × 10-4 = x
    0.065 = x = PNO

  5. Is your answer plausible?

    Check your answer by substituting the values back into the equation:
    PNOCl = 0.657 atm
    PNO = 0.065 atm
    PCl2 = ½ × 0.065 = 0.0325 atm

    KP = P(NO)P½(Cl2)

    P(NOCl)
    KP = 0.065(0.0325)½

    0.657

    KP = 1.8 × 10-2 which is the same as the value we were given in the question so we are confident our answer is correct.

  6. State your solution to the problem:

    P(NO) = 0.065 atm

Do you know this?

Join AUS-e-TUTE!

Play the game now!

Worked Example : Converting Kc to KP

Question: For the equilibrium:

2NOCl(g) ⇋ 2NO(g) + Cl2(g)

Kc = 3.75 × 10-6 at 796°C.

Calculate KP for this reaction at this temperature.

Solution:
(Based on the StoPGoPS approach to problem solving in chemistry.)

  1. What is the question asking you to do?

    Calculate KP
    KP = ?

  2. What data (information) have you been given?

    (a) 2NOCl(g) ⇋ 2NO(g) + Cl2(g)
    (b) Kc = 3.75 × 10-6
    (c) T = 796°C
    Convert temperature in celsius to kelvin:
    T = 796 + 273 = 1069 K

  3. What is the relationship between what you know and what you need to find out?

    Write the expression to convert Kc to Kp:
    KP = Kc(RT)Δn

    Use data sheet to find value for ideal gas constant, R,
    R = 0.0821 (Ideal Gas Constant)

    Use balanced chemical equation to calculate Δn :
    Δn = (2 + 1) - 2 = 1

  4. Substitute the values into the equation and solve for KP:

    KP = Kc(RT)Δn
    KP = 3.75 × 10-6(0.0821 × 1069)1
    KP = 3.29 × 10-4

  5. Is your answer plausible?

    Use your calculated value of KP and the given value of Kc and temperature to find R.
    KP = Kc(RT)Δn
    KP/Kc = (RT)Δn
    3.29 × 10-4/3.75 × 10-6 = (RT)1
    87.73 = RT
    87.73 = R(796 + 273)
    87.73 = R × 1069
    R = 87.73/1069 = 0.0821
    Since this value of R is the same as that on the Data Sheet we are reasonably confident our answer is correct.

  6. State your solution to the problem:

    KP = 3.29 × 10-4

Do you understand this?

Join AUS-e-TUTE!

Take the test now!