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Multiple choice chemistry chemical equilibrium and acids-bases dissociation constants ionisation of weak acids and weak bases ionization constants of weak acids and weak bases

The ionisation constant of an acid, $K _a$ is the measure of strength of an acid. The $K _a$ values of acetic acid, hypochlorous acid and formic acid are $1.74 \times 10^{-5}, 3.0 \times 10^{-8}$ and $1.8 \times 10^{-4}$ respectively. Which of the following orders of pH of $0.1$mol $dm^{-3}$ solutions of these acids is correct?

  1. Acetic acid > Hypochlorous acid > Formic acid

  2. Hypochlorous acid > Acetic acid > Formic acid

  3. Formic acid > Hypochlorous acid > Acetic acid

  4. Formic acid > Acetic acid > Hypochlorous acid
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

$K _a$ is a measure of the strength of the acid i.e., larger the value of $K _a$, the stronger is the acid.
Thus, the correct order of acidic strength is
$HCOOH > CH _3COOH > HClO$
Stronger the acid, lesser will be the value of pH. Hence, the correct order of pH is $HClO > CH _3COOH > HCOOH$.

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 $S _0, S _1, S _2$ and $S _3$ are the solubilities in water of $AgCl$, $0.01 \,M \,CaCl _2, 0.01 \,M \,NaCl$ and $0.5 \,M \,AgNO _3$ solutions, respectively, then which of the following is true?

  1. $S _0 > S _2 > S _1 > S _3$
  2. $S _0 = S _2 = S _1 > S _3$
  3. $S _3 > S _1 > S _2 > S _0$

  4. none of these
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

The solubility of $AgCl$ or its ion formation will depend inversely on the concentration

$S _0=H _2O$
$S _1=0.01\ M\ CaCl _2$
$S _2=0.01\ M\ NaCl$
$S _3=0.05\ M\ AgNO _3$
$H _2O$ is dilute solution and least concentrated hence have maximum solubility of $AgCl$ in it. Out of $NaCl$ and $CaCl _2$, the solubility of $NaCl$ is high due to loess number of ions produced from $NaCl$
as compare to $CaCl _2$. More the ion produced lesser is the solubility of coming salt.
So, the Correct order is $S _0 > S _2 > S _1 > S _3$
We know that concentration of common ion $\alpha \dfrac{1}{solubility}$. The order of solubility of $AgCl : S _1 > S _3 > S _2 > S _4$

Multiple choice chemistry chemical equilibrium and acids-bases dissociation constants ionisation of weak acids and weak bases ionization constants of weak acids and weak bases

The $[H^+]$ of a resulting solution that is $0.01\space M$ acetic acid $(K _a = 1.8\times10^{-5})$ and $0.01\space M$ in benzoic acid $(K _a = 6.3\times10^{-5})$:

  1. $9\times10^{-4}$
  2. $81\times10^{-4}$
  3. $9\times10^{-5}$
  4. $2.8\times10^{-3}$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

For a mixture of two weak acids, [H+] = sqrt(Ka1*C1 + Ka2*C2). [H+] = sqrt(1.8e-5 * 0.01 + 6.3e-5 * 0.01) = sqrt(0.81e-6) = 0.9e-3 = 9e-4 M.

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

$Ag _3 PO _4$ would be least soluble at 25$^o$C in

  1. 0.1 M $AgNO _3$
  2. 0.1 M $HNO _3$

  3. pure water
  4. 0.1 M $Na _3PO _4$

  5. solubility in (a), (b), (c) or (d) is not different
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

$Ag _3 PO _4$ is a weak electrolyte and $AgNO _3$ is a strong electrolyte containing common ion $(NO _3^-ion)$. Thus common ion effect is observed and the solubility of $Ag _3 PO _4$ is suppressed.
Hence, $Ag _3 PO _4$ is least soluble in 0.1M $AgNO _3$.

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 solubility of $AgI$ in $NaI$ solution is less than that in pure water because

  1. $AgI$ forms complex with $NaI$

  2. Of common ion effect
  3. Solubility product of $AgI$ is less than that of $NaI$

  4. The temperature of the solution decreases
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

The common ion presence in the solution decrease the solubility of a given sparingly soluble compound. So, solubility of AgI in NaI solution is less.

Multiple choice chemistry chemical equilibrium and acids-bases dissociation constants ionisation of weak acids and weak bases ionization constants of weak acids and weak bases

The ionic strength of $C{H} _{3}CO{O}^{-}$ ion in $0.1\ M$ $C{H} _{3}COOH$ solution having ${K} _{a}= 1.8\times {10}^{-5}$ is

  1. $0.1$
  2. $0.05$
  3. $6.7\times {10}^{-4}$
  4. $1.34\times {10}^{-3}$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

$C{H} _{3}COOH\rightleftharpoons C{H} _{3}CO{O}^{-} + {H}^{+}$


$[C{H} _{3}CO{O}^{-}]= C\alpha =C\sqrt {\frac{K _a}{C}}=\sqrt {K _aC} =\sqrt {1.8\times {10}^{-5}\times 0.1} =1.34\times {10}^{-3}$

Ionic strength of $C{H} _{3}CO{O}^{-}=\dfrac{1}{2}n\ [CH _3COO^-]{Z}^{2}=\dfrac{1}{2}\times 1\times 1.34\times {10}^{-3}\times (-1)^2=6.7\times {10}^{-4}$

where, n = no. of ions and Z= charge on the ion.

Option C 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

Simultaneous solubility of $AgCNS\ (a)$ and $AgBr\ (b)$ in a solution of water will be

${K} _{{sp} _{(AgBr)}}=5\times {10}^{-13}$ and ${K} _{{sp} _{(AgCNS)}}={10}^{-12}$ 

  1. $a=4.08\times {10}^{-7}mol$ ${litre}^{-1}$; $b=8.16\times {10}^{-7}$ $mol$ ${litre}^{-1}$
  2. $a=4.08\times {10}^{-7}mol$ ${litre}^{-1}$; $b=4.08\times {10}^{-7}$ $mol$ ${litre}^{-1}$
  3. $a=8.16\times {10}^{-7}mol$ ${litre}^{-1}$; $b=4.08\times {10}^{-7}$ $mol$ ${litre}^{-1}$

  4. None of these
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Suppose solubility of $AgCNS$ and $AgBr$ in a solution are $a$ and $b$ respectively.

$AgCNS(s)\rightleftharpoons \underset { a+b }{ { Ag }^{ + }(aq) } +\underset { a }{ { CNS }^{ - }(aq) } $

$AgBr(s)\rightleftharpoons \underset { a+b }{ { Ag }^{ + } } +\underset { b }{ { Br }^{ - } } $

$\therefore$ $[{Ag}^{2+}]=(a+b); [CN{S}^{-}]=a$ and $[{Br}^{-}]=b$

For $AgCNS:{ K } _{ { sp } _{ AgCNS }\quad  }=\left[ { Ag }^{ + } \right] \left[ { CNS }^{ - } \right] $

or $1\times {10}^{-12}=(a+b)(a)......(i)$

For $AgBr: { K } _{ { sp } _{ AgBr }\quad  }=\left[ { Ag }^{ + } \right] \left[ { Br }^{ - } \right] $

or $5\times {10}^{-13}=(a+b)(b)........(ii)$

By Eqs $(i)$ and $(ii)$, we get

$\cfrac{a}{b}=\cfrac{{10}^{-12}}{5\times {10}^{-13}}=2$  ($a=2b$)

$\therefore $ By Eq $(i)$

$(2b+b)(2b)=1\times {10}^{-12}$

$\therefore$ $6{b}^{2}=1\times {10}^{-12}$

or $b=4.08\times {10}^{-7}$ $mol$ ${litre}^{-1}$

Similarly by Eq $(ii)$

$[a+(a/2)]a=1\times {10}^{-12}$

$a=8.16\times {10}^{-7}$ $mol$ ${litre}^{-1}$

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

Addition of ${NH} _{4}Cl$ does not effect the $pH$ of solution of ${NH} _{4}OH$. 

  1. True

  2. False

  3. Ambigous

  4. None
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 ${NH} _{4}Cl$ reduces dissociation of ${NH} _{4}OH$ and because of that its pH will decreases.

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 $Hg{I} _{2}$ in water decreases in presence of $KI$.

State whether the given statement is true or false.


  1. True

  2. False
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation
$Hg{I} _{2}$ forms soluble complex with $KI$
$2KI+Hg{I} _{2}\longrightarrow {K} _{2}Hg{I} _{4}$

So solubility of $Hg{I} _{2}$ in water increases in presence of $KI$.
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: AgCl will not dissolve in a concentrated solution.
Reason: The chloride ions from NaCl suppress the solubility of AgCl.

  1. Both Assertion and Reason are true and Reason is the correct explanation of Assertion

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

  3. Assertion is true but Reason is false

  4. Assertion is false but Reason is true

  5. Both Assertion and Reason are false
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Assertion: AgCl will not dissolve in a concentrated NaCl solution.
Reason: The chloride ions from NaCl suppress the solubility of AgCl.
This is an example of a common ion effect. The chloride ions are common ions.
Both Assertion and Reason are true and Reason is the correct explanation of Assertion.