The p-Block Elements Test

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The p-Block Elements Test - 58

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Weekly Quiz Competition

Question 1
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Which is not true about borax ?

A
It is a useful primary standard for titrating against acids

B
One mole of borax contains $$4B - O - B$$ bonds

C
Aqueous solution of borax can be used as buffer

D
It is made up of two triangular $$BO_{3}$$ units and two tetrahedral $$BO_{4}$$ units

Solution

In borax $$(Na_{2}B_{4}O_{7} . 10H_{2}O),$$ among $$10$$ water molecules $$2$$ molecules are part of structure, i.e., exists $$Na_{2}[B_{4}O_{5} (OH)_{4}] . 8H_{2}O$$
$$Na_{2}[B_{4}O_{5}(OH)_{4}] . 8H_{2}O + 2HCl \rightarrow 2NaCl + 4H_{3}BO_{3} + 5H_{2}O$$
Methyl orange $$(pH = 3.7)$$ is used to detect end point. Aq. solution of borax acts as buffer, as borax is salt of strong base $$NaOH$$ and weak acid $$H_{3}BO_{3}.$$

Hence, Option "B" is the correct answer.
Question 2
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Directions For Questions

$$A$$ is a white crystalline solid. Its aqueous solution is alkaline in nature. It is used in water softening. On heating, it swells up to form a puffy mass, $$B$$. Strong heating of $$B$$ gives $$C$$. Heating of $$C$$ with nickel oxide gives a brown bead, $$D$$.

...view full instructions

The aqueous solution of 'A' is alkaline due to

A
the presence of $$Ca^{2+}$$ ions

B
the presence of $$H_{3}BO_{3}$$

C
hydrolysis of $$B_{4}O_{7}^{2-}$$

D
hydrolysis of $$CO_{3}^{2-}$$

Solution

A is borax or sodium tetraborate $$(Na_2B_4O_7.10H_2O)$$.

One mole of A contains 10 moles of water of crystallization.

It is a white crystalline solid and is used in water softening. Its aqueous solution is alkaline due to the formation of sodium hydroxide upon hydrolysis of $$Na_2B_4O_7$$. The reaction is as follows:

$$Na_2B_4O_7+7H_2O \rightleftharpoons 2NaOH + 4 H_3BO_3$$
Question 3
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Which one of the following statements is incorrect about the structural properties of $$ BF_{3}$$?

A
The unsual shortness of $$B-F$$ bonds is due to $$p\pi - p\pi $$ interaction between $$B$$ and $$F$$ atoms

B
All the three $$B-F$$ bond lengths are equal

C
Double bond is delocalized giving three resonance structures.

D
Due to back bonding, $$BF_{3}$$ and $$BCl_{3}$$ are not isostructural molecules

Solution

$$B{ F }_{ 3 }$$ has a trigonal planar structure, all three $$B-F$$ bonds lie in the same plane. Thus, the p orbitals of Boron and Fluorine become parallel.
p-orbital of Boron is empty while fluorine has a lone pair. Fluorine donates its lone pair and this is called back bonding, or $$p\pi -p\pi $$ bond. Resonating structures can be obtained.
Since the $$B{ F }_{ 3 }$$ molecule is highly symmetrical, it has no dipole moment. All bonds are of equal length, thus option (B) is correct.
In Boron trihalides, the length of $$B-F$$ bond would be shorter than expected for a single bond, due to $$p\pi -p\pi$$ bonding.
$$B{ Cl }_{ 3 }$$ is also a trigonal planar molecule. Back bonding does not change the structure of the molecule. It simply decreases bond length $$B-X$$ and decreases Lewis acidity of $$B{ X }_{ 3 }$$.
Thus, option $$(D)$$ is incorrect.
Question 4
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The tendency to show inert pair effect follows the order :

A
$$Ga< Al< In< Tl< B$$

B
$$B< Al< Ga< In< Tl$$

C
$$B=Al=Ga< In< Tl$$

D
$$Tl< In< Ga< Al< B$$

Solution

Down the group inert pair effect increases so the correct order is $$B < Al < Ga < In < Tl$$.
Question 5
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Which of the following is incorrect regarding the structural features of borax?

A
It has two tetrahedral and two triangular units

B
All the boron atoms are not in same hybrid state

C
Each boron atom of a pair is tetravalent

D
Only one boron atom has a lone pair of electrons

Solution

In the structure of borax, none of the boron atoms have lone pairs of electrons.
Question 6
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Order of stability of the ions of Ge, Sn and Pb is:

A
$$Ge^{2+} < Sn^{2+} < Pb^{2+}$$

B
$$Ge^{4+} < Sn^{4+} < Pb^{4+}$$

C
$$Sn^{4+} < Sn^{2+}$$

D
$$Pb^{2+} < Pb^{4+}$$

Solution

On moving down group IV A, the stability of the +2 oxidation state increases due to the inert pair effect which becomes more pronounced in heavier elements.

Hence, the correct order of the stability of oxidation states is $$Ge^{2+} < Sn^{2+} < Pb^{2+}$$.

So, the correct option is ' A'.
Question 7
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Consider the following reactions:

i)$$Cr_{2}O_{3}+2Al\rightarrow Al_{2}O_{3}+2Cr+heat$$
ii)$$Al_{2}O_{3}+2Cr\rightarrow Cr_{2}O_{3}+2Al+heat$$
iii)$$2Al+6NaOH\rightarrow 2Na_{3}AlO_{3}+3H_{2}$$

Out of these, the possible reactions are:

A
i and iii

B
ii and iii

C
i and ii

D
i , ii and iii
Question 8
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The number of $$-OH$$ units directly bonded to boron atoms in borax is :

A
10

B
4

C
5

D
3

Solution

As can be seen from the image 4 −OH units are directly bonded to boron atoms in borax.
Question 9
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Match the ores from List-I with their corresponding formulae from List-II.

List -I List -II
A. Borax 1) $$Al_{2}O_{3}.2H_{2}O$$
B. Razorite 2) $$Ca_{2}B_{6}O_{11}.5H_{2}O$$
C. Bauxite 3) $$Na_{2}B_{4}O_{7}.4H_{2}O$$
D.Colemanite 4) $$Na_{2}B_{4}O_{7}.10H_{2}O$$

A
A-3, B-4, C-2, D-1

B
A-2, B-3, C-4, D-1

C
A-4, B-3, C-1, D-2

D
A-1, B-2, C-3, D-4

Solution

Borax: $$Na_{2} B_{4} O_{7} .10H_{2} O $$

Razorite: $$Na_{2} B_{4} O_{7} .4H_{2} O $$

Bauxite: $$Al_{2} O_{3} .2H_{2} O $$

Colemanite: $$Ca_{2} B_{6} O_{11} .5H_{2} O $$

So, the correct option is 'C'.
Question 10
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An alkyl halide reacts with a group 14 element $$Y$$ at 570K with $$Cu$$ as a catalyst and produces a dialkyl chloro compound $$Z$$. The compound $$Z$$ on hydrolysis gives another compound which is a strong water repellent and quite inert chemically. The dioxide of $$Y$$ is acidic in nature. The alkyl halide can also be obtained from methane after monosubstitution. Identify $$Y$$ and $$Z$$.

A
$$Y = Si$$ and $$Z = (CH _{3})_{2} SiCl_{2}$$

B
$$Y = Si$$ and $$Z = CH_{3}Cl$$

C
$$Y = C$$ and $$Z = SiCl_{4} $$

D
$$Y = Si$$ and $$Z = CCl_{4} $$

Solution

Monosubstitution of methane gives methyl chloride. It reacts at $$570K$$ with $$Si$$ having $$Cu$$ as catalyst to produce $$(CH _{3})_{2} SiCl_{2}$$, which is the compound $$Z$$.

$$ (CH _{3})_{2} SiCl_{2}$$ on hydrolysis gives $$ (CH _{3})_{2} Si(OH)_{2}$$, which is a strong water repellent and quite inert chemically. Also $$SiO_2$$ is acidic in nature.

Option 'A' is correct.

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The p-Block Elements Test - 58

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The p-Block Elements Test - 58

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Question 1

It is a useful primary standard for titrating against acids

One mole of borax contains $$4B - O - B$$ bonds

Aqueous solution of borax can be used as buffer

It is made up of two triangular $$BO_{3}$$ units and two tetrahedral $$BO_{4}$$ units

Solution

Question 2

the presence of $$Ca^{2+}$$ ions

the presence of $$H_{3}BO_{3}$$

hydrolysis of $$B_{4}O_{7}^{2-}$$

hydrolysis of $$CO_{3}^{2-}$$

Solution

Question 3

The unsual shortness of $$B-F$$ bonds is due to $$p\pi - p\pi $$ interaction between $$B$$ and $$F$$ atoms

All the three $$B-F$$ bond lengths are equal

Double bond is delocalized giving three resonance structures.

Due to back bonding, $$BF_{3}$$ and $$BCl_{3}$$ are not isostructural molecules

Solution

Question 4

$$Ga< Al< In< Tl< B$$

$$B< Al< Ga< In< Tl$$

$$B=Al=Ga< In< Tl$$

$$Tl< In< Ga< Al< B$$

Solution

Question 5

It has two tetrahedral and two triangular units

All the boron atoms are not in same hybrid state

Each boron atom of a pair is tetravalent

Only one boron atom has a lone pair of electrons

Solution

Question 6

$$Ge^{2+} < Sn^{2+} < Pb^{2+}$$

$$Ge^{4+} < Sn^{4+} < Pb^{4+}$$

$$Sn^{4+} < Sn^{2+}$$

$$Pb^{2+} < Pb^{4+}$$

Solution

Question 7

i and iii

ii and iii

i and ii

i , ii and iii

Question 8

10

4

5

3

Solution

Question 9

A-3, B-4, C-2, D-1

A-2, B-3, C-4, D-1

A-4, B-3, C-1, D-2

A-1, B-2, C-3, D-4

Solution

Question 10

$$Y = Si$$ and $$Z = (CH _{3})_{2} SiCl_{2}$$

$$Y = Si$$ and $$Z = CH_{3}Cl$$

$$Y = C$$ and $$Z = SiCl_{4} $$

$$Y = Si$$ and $$Z = CCl_{4} $$

Solution

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