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Chemistry

Symbols and Formulae

Chapter: Chemical Reactions and Equations

Question 1 of 2 NDA MCQ

Which one of the following is the chemical formula of Hypobromous acid?

Detailed Explanation

Correct Answer

✓ Option B is correct. Hypobromous acid is represented as HOBr.

📖 Governing Law / Core Concept

Hypobromous acid (HOBr) is a weak acid where bromine has an oxidation state of +1. This acid is used in various applications including water treatment and as an antimicrobial agent.

🧪 Step-by-Step Breakdown

The chemical formula for hypobromous acid is derived from its structure:

\[ HOBr \]

Here, H represents hydrogen, O represents oxygen, and Br represents bromine. The oxidation state of bromine in hypobromous acid can be determined by the formula: \[ \text{Oxidation state of Br} = +1 \]

🔍 Option Analysis

  • A (HBrO₄): This is the formula for perbromic acid, where Br is in the +7 oxidation state.
  • C (HBr): This represents hydrobromic acid, a strong acid with Br in the -1 oxidation state.
  • D (HBrO₃): This is the formula for bromic acid, with Br in the +5 oxidation state.

⚡ Mnemonic / Speed-Run

To remember the formula of hypobromous acid, "Hypo" contains "Ho" - which can help recall HOBr. The order of the acids can also assist: HBr, HOBr, HBrO₃, HBrO₄ from least to most oxygen.

6. Visual Suggestion

6. Visual Suggestion

A diagram comparing the structural formulas of hypobromous acid with other bromine acids would illustrate the differences in oxidation states and compositions.

📖 Factual Verification & Reference

📖 Factual Verification & Reference:

Verified against NCERT Class 11 Chemistry, Chapter on Acids and Bases.

Question 2 of 2 NDA MCQ

Which one of the following is the chemical formula of Plaster of Paris?

Detailed Explanation

Correct Answer

✓ Option A is correct. The chemical formula for Plaster of Paris is \( \text{CaSO}_4 \cdot \frac{1}{2} \text{H}_2\text{O} \).

📖 Governing Law / Core Concept

The chemical formula for Plaster of Paris is derived from gypsum (\( \text{CaSO}_4 \cdot 2\text{H}_2\text{O} \)), which loses water upon heating, producing the anhydrous form.

🧪 Step-by-Step Breakdown

1. Gypsum is heated to remove water:
\[ \text{CaSO}_4 \cdot 2\text{H}_2\text{O} \xrightarrow{\Delta} \text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O} + \frac{3}{2}\text{H}_2\text{O} \]
2. The end product (Plaster of Paris) reacts with water to form solid gypsum again, demonstrating the reversible nature of the reaction:
\[ \text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O} + \frac{3}{2}\text{H}_2\text{O} \rightarrow \text{CaSO}_4 \cdot 2\text{H}_2\text{O} \]
3. Observations: The setting of plaster is an exothermic reaction, affirming practical applications in construction and art.

🔍 Option Analysis

- **Option B**: \( \text{CaSO}_4 \cdot 2\text{H}_2\text{O} \) represents gypsum, not Plaster of Paris. - **Option C**: \( \text{CaSO}_4 \cdot 5\text{H}_2\text{O} \) indicates a hydrate of calcium sulfate, which is commonly found in other forms. - **Option D**: \( \text{CaSO}_4 \cdot 4\text{H}_2\text{O} \) is not a standard hydrate of calcium sulfate.

⚡ Mnemonic / Speed-Run

Mnemonic for remembering the formula of Plaster of Paris: "Playful Pairs (P) of CaSO₄, half the water flies!"

Visual Suggestion

Visual Suggestion

A flowchart detailing the conversion of gypsum to Plaster of Paris and back, illustrating the thermal breakdown and rehydration processes.

📖 Factual Verification & Reference

📖 Factual Verification & Reference:

Verified against NCERT Class XI Chemistry, Chapter 2 (Structure of Atom).

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