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Equilibrium

Long Answer Type

251.

How many grammes of NaOH must be dissolved in one litre of the solution to give it a pH value of 12?

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252.

Calculate the pH of a solution obtained by mixing 150 mL of 0.1 N - NaOH and 150 ml of 0.2 N - HCl.

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253.
How does Arrhenius theory help in comparing the relative strengths of weak acids and weak bases?

The relative strength of weak acids: The relative strength of weak acids can be easily compared in terms of their dissociation constants. In a gaseous solution, the dissociation of weak acid says HA may be represented as,

Since water used as a solvent is present in large quantity, its concentration remains practically constant.

K_a is known as dissociation constant of acid. It is constant at a given temperature for a given acid. Higher the value of K_a, the stronger is the acid.

If we have two weak acids having the same molar concentration in aqueous solution i.e. C mol L , then

Hence the relative strength of two acids having the same molar concentration in aqueous solution may be compared in terms of the square root of their dissociation constants.

Relative strength of weak base:
The dissociation of weak base, say BOH may be represented as,

According to law of chemical equilibrium

Since water is used as a solvent, its concentration remains practically constant.

K_b is known as dissociation constant base. It is constant at a given temperature for a given base. Higher the value of K_b, stronger is the base.

Similarly, the relative strength of the weak bases may also be compared as,

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Short Answer Type

254. The dissociation constant of an acid HA is 1·6 X 10^–5. Calculate the H₃O⁺ ion concentration in its 0·01 M solution.

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255.

Arrange the following in decreasing order of acidic strength:
(i) HOCl (K_a = 3·0 × 10^–8)
(ii) HCN (K_a = 4·0 × 10^–10)
(iii) HNO₃ (K_a = 4·5 × 10^–4)
(iv) HF (K_a = 6·7 × 10^–4)

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256. Calculate the pH of a 0·08M solution of hypochlorous acid, HOCl. The ionisation constant of the acid is 2·5 × 10^–5. Determine the percent dissociation of HOCl.

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Long Answer Type

257. At 298 K, calculate the pH of 0·23 M weak acid (ionisation constant = 7·3 × 10^–6).

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Short Answer Type

258. The ionisation constants of HF, HCOOH and HCN at 298K are 6·8 × 10^–4, 1·8 × 10^–4 and 4·8 × 10^–9 respectively. Calculate the ionisation constants of the corresponding conjugate bases.

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259. The ionisation constant of acetic acid is 1·74 × 10^–5. Calculate the degree of dissociation of acetic acid in its 0·05 M solution. Calculate the concentration of acetate ion in the solution and its pH.

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260.

It has been found that the pH of a 0.01 M solution of an organic acid is 4.15. Calculate the concentration of the anion, the ionisation constant of the acid and its pK_a.

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