Tag: further aspects of equilibria

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Questions Related to further aspects of equilibria

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

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 ($log 2 = 0.3$)

  1. $2 \times 10^{-5}$
  2. $1 \times 10^{-5}$
  3. $4 \times 10^{-5}$
  4. $5 \times 10^{-5}$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Using the Henderson-Hasselbalch equation for a buffer (weak base + salt), pOH = pKb + log([salt]/[base]). After 5ml of HCl, 5ml of BOH is converted to B+, leaving 35ml BOH. After 20ml, 20ml BOH is converted to B+, leaving 20ml BOH. Solving the two equations for pKb yields 4.7, corresponding to Kb = 2e-5.

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

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

  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
Reveal answer Fill a bubble to check yourself
A,B Correct answer
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 $pH = -log [ H^+]$

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

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

  1. $BaCl _{2}+Ba(NO _{3}) _{2}$
  2. $NaCl+HCl$
  3. $NH _{4}OH+NH _{4}Cl$
  4. $None$
Reveal answer Fill a bubble to check yourself
C Correct answer
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).

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

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 :

  1. $60\%$
  2. $83.5\%$
  3. $46.7\%$
  4. $60.23\%$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

The van't Hoff factor i = (Calculated MW) / (Observed MW) = 170 / 92.64 = 1.835. For AgNO3, i = 1 + alpha(n-1). Since n=2, i = 1 + alpha. Thus, alpha = 0.835 or 83.5%.

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

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

  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
Reveal answer Fill a bubble to check yourself
A Correct answer
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.

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

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

  1. $NaCl$
  2. $HCI$
  3. $KCI$

  4. Unpredicatble
Reveal answer Fill a bubble to check yourself
B Correct answer
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.

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

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

  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
Reveal answer Fill a bubble to check yourself
D Correct answer
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 $pH.$

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

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

  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
Reveal answer Fill a bubble to check yourself
B Correct answer
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.

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

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.

  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
Reveal answer Fill a bubble to check yourself
B Correct answer
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)

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

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

  1. increases

  2. decreases

  3. remains the same

  4. cannot predict
Reveal answer Fill a bubble to check yourself
B Correct answer
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.