BPSC PGT Chemistry: Important Compounds of Boron

This set of BPSC PGT Chemistry Multiple Choice Questions (MCQs) is designed to cover the topic “p-Block Elements – Important Compounds of Boron.” It includes essential questions that help in understanding the properties, structure, and applications of various boron compounds, aiding in effective exam preparation.

1. What do we get upon heating or orthoboric acid at last?
   a) Metaboric acid
   b) Tetraboric acid
   c) Borax
   d) Boron trioxide

Answer: (d)
Explanation: When orthoboric acid is heated to 273 K, it forms metaboric acid. Further heating metaboric acid to 473 K yields tetraboric acid. When tetraboric acid is heated to red-hot temperatures, it decomposes into boron trioxide (boric anhydride) as the final product.

2. The complete combustion of diborane is ____________
   a) endothermic
   b) exothermic
   c) there is no change in energy
   d) depends on the reaction

Answer: (b)
Explanation: One of the key uses of diborane is as a propellant in rockets. Its complete combustion releases a significant amount of energy, making it highly exothermic. However, achieving complete combustion in rockets is challenging due to the formation of boron monoxide.

3. Does Boron occur in a free state?
   a) No
   b) Yes
   c) Sometimes
   d) May not be

Answer: (a)
Explanation: Boron does not exist in its free state; instead, it is found in compounds such as borax, kernite, and orthoboric acid. Elemental boron can be obtained through various methods, including the reduction of boric oxide with highly electropositive metals or the reaction of boron halides with hydrogen.

4. The chemical name of borax is a ____________
   a) sulphur tetraborate decahydrate
   b) sodium tetraborate decahydrate
   c) sodium pentaborate nano hydrate
   d) sodium hexa borate hexahydrate

Answer: (b)
Explanation: The chemical name of borax is sodium tetraborate decahydrate, with the formula Na₂B₄O₇·10H₂O. It consists of two sodium atoms, four boron atoms, seven oxygen atoms, and ten water molecules.

5. Diborane is a ____________
   a) monomer
   b) dimer
   c) trimer
   d) polymer

Answer: (b)
Explanation: Borane does not exist in its monomeric form; instead, it forms a dimer known as diborane. Diborane has a complex structure with sp³ hybridization. It features unique banana bonds, which are three-center, two-electron bonds.

6. What is the colour of borax bead in the basic radical chromium test?
   a) White
   b) Brown white
   c) Green
   d) Blue

Answer: (c)
Explanation: The borax bead test is used to detect colored basic radicals. When borax is heated, it decomposes into sodium metaborate and boric anhydride, forming a glassy bead. This bead reacts with basic radicals to produce characteristic colors. For example, chromium imparts a green color.

7. Tincal is ____________
   a) alum
   b) aluminium
   c) boric acid
   d) borax

Answer: (d)
Explanation: Tincal, also known as the natural and pure form of borax, has the chemical formula Na₂B₄O₇·10H₂O. Since boron does not exist in its free state, tincal serves as an important ore of boron, occurring as hydrated sodium borate.

8. Orthoboric acid is a weak monobasic Lewis acid.
   a) True
   b) False

Answer: (a)
Explanation: When one mole of orthoboric acid reacts with two moles of water, it forms a boron hydroxide anion and one hydronium ion. This reaction confirms that orthoboric acid is a weak monobasic acid as well as a Lewis acid, making the statement valid.

9. What is the chemical formula of the mineral colemanite?
   a) CaB6O.5H2O
   b) Na2B4O7.10H2O
   c) Ca2B6O.5H2O
   d) Na4O7.10H2O

Answer: (c)
Explanation: The chemical formula of colemanite is Ca₂B₆O₁₁·5H₂O. Borax naturally occurs as tincal in dried-up lakes. It can also be obtained by boiling colemanite with a sodium carbonate solution.

10. Which of the following salt has an outcome of blue colour as a result of a borax bead test?
    a) Nickel sulphate
    b) Cobalt sulphate
    c) Ferrous sulphate
    d) Chromium sulphate

Answer: (b)
Explanation: When cobalt sulfate is heated, it decomposes into cobalt oxide and sulfur trioxide. The cobalt oxide then reacts with boric anhydride, forming cobalt metaborate, which produces a blue-colored bead. The bead test colors for other metals are: iron (green), chromium (green), and nickel (brown).

11. What is inorganic benzene?
    a) Boron
    b) Borax
    c) Boroline
    d) Borazole

Answer: (d)
Explanation: Borazole, also known as borazine or inorganic benzene, has the chemical formula B₃N₃H₆. It is a colorless liquid with a six-membered ring consisting of alternating boron and nitrogen atoms, resembling the structure of benzene, which is why it is referred to as inorganic benzene.

12. In order to prepare borazine, we need by diborane and Ammonia in the ratio ____________
    a) 5:9
    b) 1:2
    c) 22:23
    d) 3:4

Answer: (b)
Explanation: Borazine is synthesized from diborane. When three moles of diborane react with six moles of ammonia upon heating, the reaction produces two moles of borazine (inorganic benzene) and twelve moles of hydrogen gas. This indicates that diborane and ammonia react in a 1:2 ratio.

13. Is inorganic benzene reactive than Benzene?
    a) Yes
    b) No
    c) Maybe
    d) Cannot say

Answer: (a)
Explanation: Yes, inorganic benzene (B₃N₃H₆), also known as borazine or borazole, is more reactive than benzene. This increased reactivity is due to the partial localization of electron pairs within the borazine ring, making it less stable compared to benzene.