Tag: ionization constants of weak acids and weak bases

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

$Ca{SO} _{4}$ is somewhat soluble in water.
$I$. When ${H} _{2}{SO} _{4}$ is added to a solution of $Ca{SO} _{4}$, the solubility of the $Ca{SO} _{4}$ will be increased.
$II$. The addition of ${H} _{2}{SO} _{4}$ will lower the $pH$ of the solution.

  1. Statement $I$ is true, Statement $II$ is true
  2. Statement $I$ is true, Statement $II$ is false
  3. Statement $I$ is false, Statement $II$ is true
  4. Statement $I$ is false, Statement $II$ is false
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

When $H _2SO _4$ is added to a $CaSO _4$ solution, due to common ion effect, the solubility of $CaSO _4$ decreases. Further, as the acid concentration increases, the $pH$ of the solution lowers due to addition of $H _2SO _4$.

Hence, statement $I$ is false, statement $II$ is true.

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

What is the common ion effect?

  1. When the addition of an ion common to two solutes causes precipitation or reduces ionization.

  2. When a molecule is added to a solution and causes precipitation or reduces ionization

  3. When the subtraction of an ion common to two solutes causes precipitation or reduces ionization.

  4. The effect of adding ions to a solution.

  5. When any ion is added to a solution and causes precipitation or reduces ionization.
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

The common ion effect is observed when the addition of an ion common to two solutes causes precipitation or reduces ionization.
Thus, if to a solution of a weak electrolyte, a solution of strong electrolyte is added which furnishes an ion common to that furnished by the weak electrolyte, the ionization of the weak electrolyte is suppressed.

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 monoprotic acid in $1.00M$ solution is $0.001$% ionised. The dissociation constant of acid is:

  1. ${ \alpha }^{ 2 }C+\alpha K-K=0$
  2. ${ \alpha }^{ 2 }C-\alpha K-K=0$
  3. ${ \alpha }^{ 2 }C-\alpha K+K=0$
  4. ${ \alpha }^{ 2 }C+\alpha K+K=0$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

According to Ostwald's dilution law


$K=\cfrac { { \alpha  }^{ 2 }C }{ \left( 1-\alpha  \right)  } $

On solving, we get


$\alpha^2C+\alpha K-K=0$


Hence, the correct option is $\text{A}$

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

The ionisation constant of acetic acid is $1.8\times { 10 }^{ -5 }$.The concentration at which it will be dissociated to $2$% is:

  1. $1M$
  2. $0.045M$
  3. $0.018M$
  4. $0.45M$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

When acetic acid is dissolved in water, it partially dissociates (2%).

Ionisation constant $ K _a=1.87\times 10^{-5}$
$K _a=\cfrac{C\alpha \times C\alpha}{C(1-\alpha)}$

We assume $\alpha\sim  0\Rightarrow 1-\alpha\sim 1$
$\Rightarrow 1.87\times 10^{-5}=\cfrac{C\alpha^2}{1}={C\times 0.02\times 0.02}$
$\Rightarrow C = \cfrac{1.87\times 10^{-5}}{0.02^2}$
$=0.0467M$
Correct answer is option-B.

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 pair will show common ion effect?

  1. $Ba{ Cl } _{ 2 }+Ba{ \left( { NO } _{ 3 } \right) } _{ 2 }$
  2. $NaCl+HCl$
  3. ${ NH } _{ 4 }OH+{ NH } _{ 4 }Cl$
  4. $AgCN+KCN$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Those ions will show common ion effect in which one or both them does not dissociate completely.


$NaCl$ and $HCl$ dissociate completely as one of them is acid and other is salt of strong acid and strong base.


$AgCN$ and $KCN$ dissociate completely as one of them is acid and other is salt of strong acid and strong base.

NH4OH is weak base to which NH4Cl is added which provides common $NH _4^{+}$ion dissociation of NH4OH is suppressed.

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 dissociation constants of two acids $ H{ A } _{ 1 }$ and $H{ A } _{ 2 }$ are $3.0\times { 10 }^{ -4 }$ and $1.8\times { 10 }^{ -5 }$ respectively. The relative strengths of the acids will be:

  1. $1:4$
  2. $4:1$
  3. $1:16$
  4. $16:1$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation
The dissociation constants of $HA _1$ and $HA _2$ are $3\times10^{-4} $ and $1.8\times10^{-5}.$

The strength of an acid is directly proportional to square root of dissociation constants of acids. So relative strength of the given acids are:
$\dfrac { { (Acidic\ Strength }) _{ HA _1 } }{ { (Acidic\ Strength }) _{ HA _2} } =\dfrac { \sqrt { { (Dissociation\ Constant }) _{ HA _1} }  }{ \sqrt { { (Dissociation\ Constant }) _{ HA _2 } }  } $

 Relative Acidic Strength= $\dfrac { { (Acidic Strength }) _{ HA _1 } }{ { (Acidic Strength }) _{ HA _2 } } =\dfrac { \sqrt { 3.0\times { 10 }^{ -4 } }  }{ \sqrt { 1.8\times { 10 }^{ -5 } }  } =4.08=4(approx)$

relative strengths of acids will be $4:1.$
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 pairs will show common ion effect?

  1. Barium chloride + barium sulphate

  2. Silver cyanide + potassium nitrite

  3. Ammonium hydroxide + ammonium chloride

  4. Sodium chloride + hydrogen chloride
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

The common ion effect occurs when a strong electrolyte and a weak electrolyte share a common ion. Ammonium hydroxide (weak base) and ammonium chloride (strong salt) share the NH4+ ion.

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

When strong base $(NaOH)$ is added to the weak acid (acetic acid, ${ CH } _{ 3 }COOH$), then dissociation of acetic acid increases; this effect is known as:

  1. Common ion effect

  2. Reverse ion effect

  3. Saltation effect

  4. Solubility effect
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

$CH _3COOH\rightleftharpoons CH _3COO^{-}+H^+$
When $NaOH$ is added $H^+$ combines with $OH^{-}$ so concentration of $H^+$ decreases so equilibrium will shift towards right.
${ CH } _{ 3 }COOH+NaOH\longrightarrow { CH } _{ 3 }COONa+{ H } _{ 2 }O\quad $
Ionization of acetic acid will increase with the progress of its neutralization. This effect is called reverse ion effect.

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 precipitating reagent is $NH {4}Cl + NH _{4} OH$. The function of $NH _{4}Cl$ is to______

  1. increase the ionization of $NH _{4} OH$
  2. suppress the ionization of $NH _{4} OH$

  3. stabilise the hydroxides of group cations

  4. convert the ions of group third into their respective chlorides
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Common ion effect is observed when a solution of weak electrolyte is mixed with a solution of strong electrolyte, which provides an ion common to that provided by weak electrolyte.


The NH4OH is weak base it does not ionises completely. Thus due to presence of common ion NH4+ in NH4Cl, it supresses the ionisation of weak base NH4OH in order to decrease the OH- concentration so that higher group cations will not get precipitated.

Thus the pair $NH _{4} OH + NH _{4} Cl$ shows common ion effect. 

Ammonium chloride suppresses the ionization of ammonium hydroxide.

Option B is correct.

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 solubility of ${ A } _{ 2 }{ X } _{ 5 }$ is $x\ mol\ { dm }^{ -3 }$. Its solubility product is:

  1. $36{ x }^{ 6 }$
  2. $64\times { 10 }^{ 4 }{ x }^{ 7 }$
  3. $126{ x }^{ 7 }$
  4. $1.25\times { 10 }^{ 4 }{ x }^{ 7 }$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation
Let the solubility be $S.$
${ As } _{ 2 }X _{ 3 }\leftrightharpoons 2As^{ 5+ }+5X^{ 2- }\\ \quad \quad \quad   \quad 2S\quad \quad \quad 5S$
Solubility product is 
$K _{ sp }=[As^{ 5+ }]^{ 2 }\times [S^{ 2- }]^{ 5 }\\$ 
Let $[{ As } _{ 2 }S _{ 5 }]=S,[As^{ 5+ }]=2S, [S^{ 2- }]=5S\\ Ksp=(2S)^{ 2 }\times (5S)^{ 5 }=4S^{ 2 }\times 3125S^{ 5 }=12500S^{ 7 }$