Which of the following statements is-are correct for a salt of weak acid and weak base in an aqueous solution at 25- C- Here- Ka is the dissociation constant for a weak acid and Kb is the dissociation constant for a weak base-

For an aqueous solution of weak acid and weak base at 25^∘ C, nbsp; pH=7+12(pK_a pK_b) Since , pK_a= log (K_a) pK_b= log(K_b) Putting in the pH equation, pH=7 ...

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Byju's Answer

Standard XII

Chemistry

Salt of Weak Acid and Weak Base

Which of the ...

Question

Which of the following statement(s) is/are correct for a salt of weak acid and weak base in an aqueous solution at $25^{\circ} C$ .

Here, $K_{a}$ is the dissociation constant for a weak acid and $K_{b}$ is the dissociation constant for a weak base.

K_{a} > K_{b}

then the solution is neutral.

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K_{a} < K_{b}

then the solution is acidic in nature.

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K_{a} > K_{b}

then the solution is acidic in nature.

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K_{a} < K_{b}

then the solution is alkaline in nature.

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Solution

The correct option is D If $K_{a} < K_{b}$ then the solution is alkaline in nature.
For an aqueous solution of weak acid and weak base at $25^{\circ} C$ ,
$p H = 7 + \frac{1}{2} (p K_{a} - p K_{b})$
Since , $p K_{a} = - l o g (K_{a}) p K_{b} = - l o g (K_{b})$
Putting in the $p H$ equation,
$p H = 7 + \frac{1}{2} (- l o g (K_{a}) + l o g (K_{b})) p H = 7 + \frac{1}{2} (l o g (K_{b}) - l o g (K_{a}))$
When, $K_{a} > K_{b} l o g (K_{a}) > l o g (K_{b}) \Rightarrow p H < 7$ .
$∴$ Solution is acidic in nature.

When,
$K_{a} < K_{b} l o g (K_{a}) < l o g (K_{b}) \Rightarrow p H > 7$
Solution is alkaline in nature.

Theory :
For a weak acid and weak base
$- l o g [H^{+}] = - \frac{1}{2} l o g K_{w} - \frac{1}{2} l o g K_{a} + \frac{1}{2} l o g K_{b}$
$p H = \frac{1}{2} (p K_{w} + p K_{a} - p K_{b})$
valid only if h<0.1 or 10%
$p H = 7 + \frac{1}{2} (p K_{a} - p K_{b})$ at $25^{o} C$
Since expression for $[H^{+}]$ or pH is independent of concentration of salt.
So, if $K_{a} = K_{b} = p H = 7$ it is a neutral solution.
If $K_{a} > K_{b} = p H < 7$ it is an acidic solution.
If $K_{a} < K_{b} = p H > 7$ it is a basic solution.

$p H = \frac{1}{2} (p K_{w} + p K_{a} - p K_{b})$
This formula is always valid for any $K_{a} a n d K_{b}$ at any temperatures and for any values of h.
pH is independent of the concentration of the salt solution.

Suggest Corrections

Similar questions

Q. Which of the following statement(s) is/are correct for a salt of weak acid and weak base in an aqueous solution at

25^{\circ} C

.

Here,

K_{a}

is the dissociation constant for a weak acid and

K_{b}

is the dissociation constant for a weak base.

Q. Which of the following describes the correct relation for hydrolysis constant

(K_{h})

for a salt of weak acid and weak base at

25^{\circ} C ?

Q. Which of the following describes the correct relation for

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for an aqueous salt solution of weak acid and weak base at

25^{\circ} C ?

Q. At

25^{o} C,

the dissociation constant

(K_{a})

of a weak monoprotic acid,

H A

is numerically equal to the dissociation constant

(K_{b})

of its conjugate base,

A^{-} .

Which of the following statements (s) is/are correct?