Tag: the p-block elements - group 13

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Questions Related to the p-block elements - group 13

Multiple choice chemistry the p-block elements - group 13 study of orthoboric acid some important compounds of boron study of boron

Which of the following statements about $H _{3}BO _{3}$ is not correct?

  1. It is prepared by acidifying an aqueous

    solution of borax
  2. It has a layer structure in which planar $BO _{3}$ units are joined by hydrogen bonds

  3. It does not act as proton donor but acts as

    Lewis acid by accepting hydroxyl ion

  4. It is a strong tribasic acid.
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

It is a monobasic lewis acid. It accepts an electron pair from $OH^{-}$.
$B(OH) _{3} + 2H _{2} O \rightleftharpoons [B(OH) _{4} ]^{-} +H _{3} O^{+} $

Multiple choice chemistry the p-block elements - group 13 study of orthoboric acid some important compounds of boron study of boron

$H _{3}BO _{3}\overset{T _{1}}{\rightarrow}X\overset{T _{2}}{\rightarrow}Y\overset{red:hot}{\rightarrow}B _{2}O _{2}$


 if $T _{1}< T _{2}$ then X and Y respectively are-

  1. $X=$ Metaboric acid and $Y=$ Tetraboric acid
  2. $X=$ Tetraboric acid and $Y=$ Metaboric acid
  3. $X=$ Borax and $Y=$ Metaboric acid
  4. $X=$ Tetraboric acid and $Y=$ Borax
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation
Effect of temperature at $100^{\circ}C \,,\,H _{3}BO _{3}$ losses water and convert into metaboric acid.

$H _{3}BO _{3}\xrightarrow{100^{\circ}C}HBO _{2}+H _{2}O$

metaboric acid form tetraboric acid on heating at $160^{\circ}C$

$4HBO _{2}\xrightarrow{160^{\circ}C}H _{2}B _{4}O _{7}+H _{2}O$

On strong heating, $B _{2}O _{3}$ is produced

$H _{2}B _{4}O _{7}\rightarrow 2B _{2}O _{3}+H _{2}O$

Hence, option $A$ is correct.
Multiple choice chemistry the p-block elements - group 13 study of orthoboric acid some important compounds of boron study of boron

The states of hybridisation of boron and oxygen atoms in boric acid $(\mathrm{H} _{3}\mathrm{B}\mathrm{O} _{3})$ are respectively:

  1. $\mathrm{s}\mathrm{p}$ and $\mathrm{s}\mathrm{p}^{2}$
  2. $\mathrm{s}\mathrm{p}^{2}$ and $\mathrm{s}\mathrm{p}^{3}$
  3. sp3 and $\mathrm{s}\mathrm{p}^{2}$

  4. sp3 and sp3
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

The states of hybridisation of boron and oxygen atoms in boric acid $(\mathrm{H} _{3}\mathrm{B}\mathrm{O} _{3})$ are respectively are $\mathrm{s}\mathrm{p}^{2}$ and $\mathrm{s}\mathrm{p}^{3}$
respectively.

In boric acid the boron is surrounded by three bond pair thus it shows $sp^2$ hybridization while oxygen is surrounded by one two bond pair and two lone pairs so it shows $sp^3$ hybridization.

Multiple choice chemistry the p-block elements - group 13 study of orthoboric acid some important compounds of boron study of boron

$\mathrm{H} _{3}\mathrm{B}\mathrm{O} _{3}$ is:


  1. Monobasic and weak Lewis acid

  2. Monobasic and weak Bronsted acid

  3. Monobasic and strong Lewis acid

  4. Tribasic and weak Bronsted acid
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

$\mathrm{H} _{3}\mathrm{B}\mathrm{O} _{3}(\mathrm{C})$ (orthoboric acid) is a weak lewis acid.
$\mathrm{H} _{3}\mathrm{B}\mathrm{O} _{3}+\mathrm{H} _{2}\mathrm{O} _{-}^{-}\mathrm{E}(\mathrm{O}\mathrm{H}) _{4}^{-}+\mathrm{H}^{\oplus}$
So as it releases one $H^{+}$ so its Monobasic and weak Lewis acid
Hence option A is correct.

Multiple choice chemistry the p-block elements - group 13 study of orthoboric acid some important compounds of boron study of boron

Boric acid is :

  1. monobasic and weak lewis acid

  2. monobasic and weak bronsted acid

  3. monobasic and strong lewis acid

  4. tribasic and strong bronsted acid
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Boric acid is monobasic acid and weak Lewis acid.


Boric acid contains $3-OH$ groups yet it acts as monobasic acid rather than tribasic acid. Because boric acid does not act as proton donor but it accepts a lone pair of electrons from $OH^{-}$ ions and forms ${B(OH) _4}^-$. Boric acid behaves as Lewis acid because it accepts a lone pair of electrons from $OH^{-}$ ions from water. 
Boric acid considered as weak acid because it does not dissociate to produce ions rather forms metaborate ion and in turn release ions.

Multiple choice chemistry the p-block elements - group 13 study of orthoboric acid some important compounds of boron study of boron

Boric acid is a:

  1. strong tribasic acid

  2. strong monobasic acid

  3. weak tribasic acid

  4. weak monobasic acid
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Boric acid is a weak, monobasic lewis of boron often used as an antiseptic, insecticide, flame retardant, neutron absorber or precursor to other chemical compounds. It has the chemical formula,$H _3BO _3$ (sometimes written $B(OH) _3$ and exists in the form of colourless crystals or a white powder that dissolves in water.

Multiple choice chemistry the p-block elements - group 13 study of orthoboric acid some important compounds of boron study of boron

On the addition of mineral acid to an aqueous solution of borax, the compound formed is:

  1. orthoboric acid

  2. boron hydride

  3. metabolic acid

  4. pyroboric acid
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Addition of mineral acid to an aqueous solution of Borax,

Orthoboric acid is formed.

$Na _2B _4O _7+2HCl+5H _2O\longrightarrow 4H _3BO _3 + 2NaCl$

Borax.        Hydrochloric      Orthoboric acid  Sodium chloride Acid

Boric acid is soluble in boiling water. When heated above $170^oC$, it dehydrates, forming metaboric acid $(HBO _2)$:

Acid melts at about $236^oC$, and when heated above about $300^o C$ further dehydrates, forming tetraboric acid, also called pyroboric acid $(H _2B _4O _7)$.

Multiple choice chemistry the p-block elements - group 13 study of orthoboric acid some important compounds of boron study of boron

An aqueous solution of boric acid is found to be weakly acidic in nature. This acidic character arises due to the following reasons.

  1. It is a protic acid which donates protons in aqueous solution.
  2. It is a Lewis acid which abstracts $OH^-$ from water and leaves $H^+$ to make the solution acidic.

  3. It gives metaboric acid when dissolves in water

  4. It is prepared by reaction of borax with sulphuric acid hence it behaves as an acid.
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Boric acid, when reacts with water, abstracts ${OH}^{-}$ ions and releases ${H _3O}^{+}$ ions.

$H _3BO _3 + H _2O \; \longrightarrow \; {H _3O}^{+} + {H _2BO _3}^{-}$
This property of boric acid makes it acidic.

Multiple choice chemistry the p-block elements - group 13 study of orthoboric acid some important compounds of boron study of boron

Which of the following compounds are formed when $BCl _3$ is treated with water?

  1. $H _3BO _3$
  2. $B _2H _6$
  3. $B _2O _3$
  4. $HBO _2$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation
$B{Cl} _3$ hydrolyzes readily to give hydrochloric acid and boric acid.

$B{Cl} _3 + 3H _2O \; \xrightarrow{hydrolyzes} \; B{(OH)} _3 + 3HCl$

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