Tag: study of boron

Questions Related to study of boron

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

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

  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.

Show answer
Correct Option: D
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^{+} $

Solid oxy acids of boron are.

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

  1. $H _{3}BO _{3}$

  2. $HBO _{2}$

  3. $H _{2}B _{4}O _{7}$

  4. All

Show answer
Correct Option: D
Explanation:

All are solids.

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


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

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

  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

Show answer
Correct Option: A
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.

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

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

  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

Show answer
Correct Option: B
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.

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


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

  1. Monobasic and weak Lewis acid

  2. Monobasic and weak Bronsted acid

  3. Monobasic and strong Lewis acid

  4. Tribasic and weak Bronsted acid

Show answer
Correct Option: A
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.

Boric acid is :

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

  1. monobasic and weak lewis acid

  2. monobasic and weak bronsted acid

  3. monobasic and strong lewis acid

  4. tribasic and strong bronsted acid

Show answer
Correct Option: A
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.

Boric acid is a:

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

  1. strong tribasic acid

  2. strong monobasic acid

  3. weak tribasic acid

  4. weak monobasic acid

Show answer
Correct Option: D
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.

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

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

  1. orthoboric acid

  2. boron hydride

  3. metabolic acid

  4. pyroboric acid

Show answer
Correct Option: A
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 $(H _2B _4O _7)$.

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

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

  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.

Show answer
Correct Option: B
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.

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

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

  1. $H _3BO _3$

  2. $B _2H _6$

  3. $B _2O _3$

  4. $HBO _2$

Show answer
Correct Option: A
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 $\text{A}$.