# 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:

Questions:

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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