Tag: further aspects of equilibria

Questions Related to further aspects of equilibria

A $40.0 ml$ solution of weak base, $BOH$ is titrated with $0.1 N - HCl$ solution. The $pH$ of the solution is found to be $10.0$ and $9.0$ after adding $5.0 ml$ and $20.0 ml$ of the acid, respectively. The dissociation constant of the base is ($log 2 = 0.3$)

chemistry further aspects of equilibria dissociation constants ionisation of weak acids and weak bases ionization constants of weak acids and weak bases

  1. $2 \times 10^{-5}$

  2. $1 \times 10^{-5}$

  3. $4 \times 10^{-5}$

  4. $5 \times 10^{-5}$

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Correct Option: A

If a salt of weak acid or base is added to a solution of its acid or base respectively, the:

chemistry further aspects of equilibria dissociation constants ionisation of weak acids and weak bases ionization constants of weak acids and weak bases

  1. dissociation of acid or base is diminished

  2. the $pH$ of the solution in case of acid increases and in case of base decreases

  3. mixing of two leads for precipitation

  4. none of the above

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Correct Option: A,B
Explanation:

Due to common ion effect, if a salt of weak acid or base is added to a solution of its acid or base respectively, the dissociation of acid or base is diminished.


As a result, concentration of hydrogen ions or hydroxide ion will change and pH of solution increases in case of acid and decreases in case of base.

As $pH = -log [ H^+]$

The common ion effects is shown by which of the following sets of solutions:-

chemistry further aspects of equilibria dissociation constants ionisation of weak acids and weak bases ionization constants of weak acids and weak bases

  1. $BaCl _{2}+Ba(NO _{3}) _{2}$

  2. $NaCl+HCl$

  3. $NH _{4}OH+NH _{4}Cl$

  4. $None$

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Correct Option: C
Explanation:

Common ion effect is normally applied in selective precipitation of ions which involves a technique of separating ions in an aqueous solution by using a reagent that precipitates one or more of the ions. addition of common ions in a solution that is already having that given ion normally leads to a formation of a precipitate. The common ion normally decreases the solubility of a slightly insoluble salt.

Its not (1) because they are both solids.

The remaining are solute solvent pairs. So all the other pairs will show common ion effect if the solute is soluble to some extent and exists in an equilibrium with the solvent. The answer given is (3).

The value of observed and calculated molecular weight of silver nitrate are $92.64$ and 170 respectively. The degree of dissociation of silver nitrate is :

chemistry further aspects of equilibria dissociation constants ionisation of weak acids and weak bases ionization constants of weak acids and weak bases

  1. $60\%$

  2. $83.5\%$

  3. $46.7\%$

  4. $60.23\%$

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Correct Option: B

In the dissociation of $NH _4OH$, if excess if $NH _4Cl$ is added before adding $NH _4OH$, the concentration of:

chemistry further aspects of equilibria dissociation constants ionisation of weak acids and weak bases ionization constants of weak acids and weak bases

  1. $NH _4^+$ ions increases and $OH^-$ ions decreases

  2. both $NH _4^+$ ions and $OH^-$ ions increases

  3. $NH _4^+$ ions decreases and $OH^-$ ions increases

  4. both $NH _4^+$ ions and $OH^-$ ions decreases

Show answer
Correct Option: A
Explanation:

From the law of Mass action, the dissociation of $NH _4OH$ takes place and we have,
$\dfrac{[NH _4^+][OH^-]}{[NH _4OH]} =K$
Ammonium chloride, a strong electrolyte, ionises almost completely as follows:
$NH _4Cl \leftrightarrow NH _4^+ + Cl^-$
So, if excess of $NH _4Cl$ is added before adding $NH _4OH$, the concentration of $NH _4^+$ ions is increased and consequently the concentration of $OH^-$ ions is decreased.

Which of the following will supress the ionisation of acetic acid in aqueous solution?

chemistry further aspects of equilibria dissociation constants ionisation of weak acids and weak bases ionization constants of weak acids and weak bases

  1. $NaCl$

  2. $HCI$

  3. $KCI$

  4. Unpredicatble

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Correct Option: B
Explanation:

Adding a common ion prevents the weak acid or weak base from ionizing as much as it would without the added common ion. The common ion effect suppresses the ionization of a weak acid by adding more of an ion that is a product of this equilibrium.

So addition of $CH _3COONa$ to $CH _3COOH$ increases the $pH$ of solution but addition of $NH _4Cl$ to $NH _4OH$ decreases the $pH$ of solution.
Here, the addition of $HCl$ acid will suppress the ionisation of acetic acid.

Assertion: $pH$ value of $HCN$ solution decreases when $NaCN$ is added to it
Reason: $NaCN$ provides a common ion $CN^{\circleddash}$ to $HCN$

chemistry further aspects of equilibria dissociation constants ionisation of weak acids and weak bases ionization constants of weak acids and weak bases

  1. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion

  2. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion

  3. Assertion is correct but Reason is incorrect

  4. Assertion is incorrect but Reason is correct

  5. Both Assertion and Reason are incorrect

Show answer
Correct Option: D
Explanation:

$(A)$ is wrong because the addition of $NaCN$ to $HCN$, due to common ion $(CN^{\circleddash})$, the degree of dissociation of $HCN$ is suppressed and hence less $[H^{\oplus}]$ and increase in $pH.$

In the third group of qualitative analysis, the precipatitating reagent is $NH _4Cl / NH _4OH$. The function of $NH _4Cl$ is to 

chemistry further aspects of equilibria dissociation constants ionisation of weak acids and weak bases ionization constants of weak acids and weak bases

  1. increase the ionisation of $NH _4OH$

  2. supress the ionisation of $NH _4OH$

  3. convert the ions of group third into their respective chlorides

  4. stabilise the hydroxides of group $III$ cations

Show answer
Correct Option: B
Explanation:

Adding a common ion prevents the weak acid or weak base from ionizing as much as it would without the added common ion. The common ion effect suppresses the ionization of a weak acid by adding more of an ion that is a product of this equilibrium.

So, addition of $CH _3COONa$ to $CH _3COOH$ increases the pH of solution but addition of $NH _4Cl$ to $NH _4OH$ decreases the pH of solution.
So, the function of $NH _4Cl$ to suppress the ionisation of $NH _4OH$ and to reduce conentration of hydroxide ion in solution.

Assertion: Due to common ion effect, the solubility of $HgI _2$ is expected to be less in an aqueous solution of KI than in water. But $HgI _2$ dissolves in an aqueous solution of KI to form a clear solution.
Reason: $I^{\circleddash}$ ion is highly polarisable.

chemistry further aspects of equilibria dissociation constants ionisation of weak acids and weak bases ionization constants of weak acids and weak bases

  1. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion

  2. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion

  3. Assertion is correct but Reason is incorrect

  4. Assertion is incorrect but Reason is correct

  5. Both Assertion and Reason are incorrect

Show answer
Correct Option: B
Explanation:

Due to common ion effect, the solubility of $HgI _2$ is expected to be less in an aqueous solution of KI than in water as
$HgI _2 + KI \longrightarrow K _2[HgI _4]$.
since $I^{\circleddash}$ ion is large sized and therefore is highly polarisable.
But (R) is not the correct explanation of (A)

In the presence of a common ion (incapable of forming complex ion), the solubility of salt _______  in solution.

chemistry further aspects of equilibria dissociation constants ionisation of weak acids and weak bases ionization constants of weak acids and weak bases

  1. increases

  2. decreases

  3. remains the same

  4. cannot predict

Show answer
Correct Option: B
Explanation:

$AB \rightarrow A^+ + B^-$
$BC \rightarrow B^+ + C^-$
Since $B^+$ is incapable of forming a complex salt it tends to decrease the solubility by Le-Chatelier's principle.