Law of Mass Action worksheet

Law of Mass Action states that, ” The rate of chemical reaction at constant temperature is directly proportional to the active mass of the reactant present at that time. If more than one reactant take part in the reaction then the rate of the reaction is proportional to the product of the active masses of the reactants.”

Here, you can find some questions related to “Law of Mass Action” and their answers:


1. Equilibrium constant for the reaction 2A (g) \leftrightharpoons B (g) + C (g) at 780 K and 10 atm pressure is 3.52 . The equilibrium constant of this reaction at 780 K and 20 atm pressure is:

(a)    5.72

(b)   11.44

(c)    28.6

(d)   3.52


2. The equilibrium constant for the reaction, HA + B \leftrightharpoons BH^+ + A^- is 100. If rate constant for forward reaction is 10^5 mol L^- s^{-1}, then rate constant for the backward  reaction is:

(a)    10^3 mol L^{-1} S^{-1}

(b)   10^6 L^{-1}S^{-1}

(c)    10^{-3}mol L^{-1} S^{-1}

(d)   10^{-5} mol L^{-1} s^{-1}


3. For the reaction: A + B \leftrightharpoons C + D, the rate constants for the forward and backward reactions are found to be 4.2. \times 10^{-2} \text{and} 3.36 \times 10^{-3} mol L^{-1} S^{-1}, respectively,  What is the equilibrium constant for the reaction?

(a)     11.5

(b)   12.5

(c)    8.

(d)   6.0


4. The rate constants for the forward and backward reactions of hydrolysis of ester are, 1.1 \times 10^{-2} \text{and} 1.5 \times 10^{-3} mol L^{-1}s^{-1} , respectively. The equilibrium constant of the reaction CH_3COOC_2H_5 + H_2O \leftrightharpoons CH_3COOH + C_2H_5OH is:

(a)    6.53

(b)   7.34

(c)    7.75

(d)   8.33


5. Reaction quotient for the reaction: N_2 (g) + 3H_2 (g) \leftrightharpoons 2NH_3 (g)

Is given by Q = \dfrac{[NH_3]^2}{[N_2][H_2]^3} from right to left, if K_c i.e., equilibrium constant:

(a)    Q < k_c

(b)   Q = 0

(c)    Q > K_c

(d)   Q = K_c


6. For the reaction; H_2 +I_2 \leftrightharpoons 2HI,  Equilibrium concentrations of  H_2, I_2 and HI are 8, 3 and 28 mol L^{-1} respectively. Equilibrium constant of the reaction is:

(a)    32.67

(b)    31.67

(c)    34.67

(d)   36.67


7. For the reaction at equilibrium N_2O_4 (g) \leftrightharpoons 2NO_2 (g) the concentrations of N_2O_4 \text{and} NO_2 \text{are} 4.8 \time10^{-2} \text{and} 1.2 \times 10^{-2} mol L^{-1} respectively. The value of Kc for reaction is:

(a)    3 \times 10^{-3}

(b)   3 \times 10^3

(c)    3.3 \times 10^2

(d)   3 \times 10^{-1}


8. What will be the value of equilibrium constant (K_1)  for the reaction, HI ( g ) \leftrightharpoons 1/2 H_2 (g) + 1/2 I_2 (g), if its value for the reaction H_2 (g) \leftrightharpoons 2HI is 64?

(a)    1/64

(b)   1/8

(c)    64

(d)   8


9. The equilibrium constant for the reactions, H_3PO_4 \leftrightharpoons H^+ + H_2PO^-_4 \hspace{2mm} \text{is}\hspace{2mm} K_1 for reaction H_2PO^-_4 \leftrightharpoons H^+ + hpo^{2-}_4 is K_2

and for reaction HPO^{2-}_4 \leftrightharpoons H^+ + PO^{3-}_4 is K_3.

The equilibrium constant (K) for H_3PO_4 \leftrightharpoons 3H^+ + PO^{3-}_4 will be:

(a)    K_1 \times K_2 \times K_3

(b)   K_1 / K_2 K_3

(c)    K_2 / K_1 K_3

(d)   K_1 + K_2 + K_3


10. At constant temperature, the equilibrium constant for the decomposition reaction; N_2O_4 \leftrightharpoons 2NO_2 is expressed by K_p / (4x^2p ) / (1- x^2) , where p = pressure, x = extent of decomposition. Which one of the following statements is true?

(a)    Kp remains constant with change in p & x

(b)   Kp increases with decrease of x.

(c)    Kp increases with increase of x

(d)   Kp increases with increase of p.


11. For a gaseous reaction: 2A B \leftrightharpoons C+ D, the partial pressures of A, B, C and D at equilibrium are 0.5, 0.8, 0.7and 1.2 atm. The value of Kp for this reaction is:

(a)    2.4 atm

(b)   6.2 atm^{-2}

(c)    4.2 atm^{-1}

(d)   8.4 atm^{-3}


12. For the reaction C (s)+CO_2(g) \leftrightharpoons 2CO(g), the partial pressures of CO_2 and CO are 4 and 8 atm respectively. The value of Kp for this reaction is:

(a)    14 atm

(b)   16 atm

(c)    18 atm

(d)   12 atm


13. In an equilibrium reaction: 2SO_2(g) + O_2(g) \leftrightharpoons 2SO_3 (g) the partial pressures SO_2O_2 and SO_3 are 0.662, 0.101 and 0.331 atm, respectively. What should be the partial pressure of oxygen if the equilibrium concentration of SO_3 \text{and} SO_2 becomes equal?

(a)    0.4 atm^{-1}

(b)   0.6 atm^{-1}

(c)    0.12 atm^{-1}

(d)   0.8atm^{-1}


1.(d)       2. (a)       3. (b)      4. (b)      5. (c)

6. (a)      7. (a)       8. (b)      9. (a)      10. (a)

11. (c)     12. (b)     13. (a)

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