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Equilibrium

Short Answer Type

291.

Calculate the degree of hydrolysis of 0.015 M solution of NH₄Cl. Given K_b for NH₄OH is 1·8 × 10^–5, K_w = 10^–14at 25°C.

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Long Answer Type

292.

Explain the terms: buffer solution and buffer capacity.

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293.

Show with an example how buffer solution resists the action of acid or base towards change in pH.
Or
Discuss the buffer action of:
(i) acidic buffer
(ii) basic buffer.

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294. How will you explain buffer action of aqueous solution of ammonium acetate?

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295.
Calculate the pH of:
(i) an acidic buffer mixture
(ii) a basic buffer mixture.
Or
Derive Henderson’s equation for an acidic and basic buffer mixture.
Or
Derive the following equation for the pH of an acidic buffer:
$pH space equals space pK subscript straight a space plus space log space fraction numerator open vertical bar Salt close vertical bar over denominator open vertical bar Acid close vertical bar end fraction$

For acidic buffer mixture: Consider an acid buffer. Let it be a mixture of HA (weak acid) and its salt with a strong base, BA. A weak acid is only ionised to a small extent.

Applying the law of chemical equilibrium, we get,

Due to the increase in the concentration of A^– ions the dissociation of the weak acid is further suppressed due to common ion effect. The concentration of weak acid molecules is taken as the concentration of HA and the concentration of the salt is the concentration of A^– ions. Thus,
[HA] = [Acid]
[BA] = [A^-] = [Salt]
Substituting the values of [HA] and [A^–] in expression (1), we have.

Taking logarithm of both the sides, we get,

This equation is known as Henderson equation. It helps in calculating the pH value of the buffer solution if the concentration of acid, as well as the salt, is known.

For basic buffer mixture: Proceeding exactly in a similar way as above, the expression for the pH of a basic buffer mixture (BOH + BA) can be obtained. BOH being a weak base dissociates to a small extent.

The salt BA being strong electrolyte dissociates almost completely.

Taking logarithm of both the sides we get,

This is also called Henderson equation. The pH of the basic buffer solution can be obtained by the following expression:

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Short Answer Type

296. Calculate the pH value of a solution obtained by mixing 0·083 moles of acetic acid and 0·091 moles of sodium acetate and making the volume 500 ml. K_a for acetic acid is 1·75 × 10^–5.

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Long Answer Type

297. Calculate the pH of a solution obtained by mixing 6·0g of acetic acid and 12·30g of sodium acetate and making the volume of solution to 500 ml. K_a for acetic acid is 1·8 × 10^–5

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298. Determine the pH of a solution obtained by mixing equal volumes of 0·015N NH₄OH and 0·15N NH₄NO₃ solutions. (K_b for NH₄OH is 1·8 × 10^–5).

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299.

Describe Ostwald’s theory of acid-base indicators.
Or
How does Ostwald’s theory explain the colour change of:
(i) Phenolphthalein
(ii) Methyl orange in acid-base titrations?

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