# Ostwald law worksheet

Ostwald law shows the relationship between the dissociation constant and degree of dissociation of a weak electrolytes.  This relationship is only approximate but is sufficiently accurate to be practically useful.

Questions:

1. Hydrogen ion concentration of 0.2 M $CH_3COOH$ which is 40% dissociated is:

(a)    0.08 M

(b)   0.12 M

(c)    0.8 M

(d)   0.4 M

2. In the reaction; $H_2S \leftrightharpoons 2H^+ + S^{2-}$ when $NH_4OH$ is added, then

(a)    concentration of $S^{2-}$ decreases

(b)   concentration of $S^{2-}$ increases

(c) $S^{2-}$ is precipitated

(d)   no action takes places

3. The degree of dissociation of 0.01 M solution of formic acid is $(Ka = 2.0 \times 10^{-4})$:

(a)    0.18

(b)   0.14

(c)    0.25

(d)   0.21

4. The ionization constant of acetic acid is $1.8 \times 10^{-5}$ What concentration of it will be dissociated to 2%?

(a)    0.018 M

(b)   1 M

(c)    0.045 M

(d)   0.18 M

5. The dissociation constants of acids HA and HB are $3.2 \times 10^{-4} \text{and} 2\times 10^{-5}$ respectively. The ratio of the strength HA and HB will be:

(a)    4 : 1

(b)   1 : 4

(c)    16 : 1

(d)   1 : 16

6. If the acids HA and HB have dissociation constants $1 \times 10^{-3} \text{and} 1 \times 10^{-5}$  respectively, then:

(a)    HA is five times stronger than HB

(b)   HA is ten times stronger than HB

(c)    HA is ten times weaker than HB

(d)   HA is five times weaker than HB

7. The Ostawald’s dilution law is expressed as:

(a) $K_a = \dfrac{C. \alpha^2}{(1- \alpha)}$

(b) $K_a = \dfrac{(1- \alpha )}{C . \alpha^2}$

(c) $K_a = \dfrac{(C. \alpha )}{(1- \alpha)}$

(d) $K_a = \dfrac{C(1- \alpha)}{\alpha^2}$

8. A 0.004 M solution of $Na_2SO_4$  is isotonic with a 0.01 M solution of glucose at the same temperature. The degree of ionizaton of $Na_2SO_4$ is:

(a)    50%

(b)   75%

(c)    25%

(d)   95%

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

6. (b)     7. (a)     8. (b)

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