Q5
1 markMCQSection A

Consider the following compounds : FeSO4\mathrm{FeSO_4} ; CuSO4\mathrm{CuSO_4} ; CaSO4\mathrm{CaSO_4} ; Na2CO3\mathrm{Na_2CO_3} The compound having maximum number of water of crystallisation in its crystalline form in one molecule is :

Chemical Reactions and Equations
Decomposition Reaction

Options

(a)FeSO4\mathrm{FeSO_4}
(b)CuSO4\mathrm{CuSO_4}
(c)CaSO4\mathrm{CaSO_4}
(d)Na2CO3\mathrm{Na_2CO_3}
Official Answer

The correct option is (d) Na2CO3\mathrm{Na_2CO_3}.

Na2CO3⋅10H2O\mathrm{Na_2CO_3 \cdot 10H_2O}water of crystallisation10 moleculesFeSO4⋅7H2O\mathrm{FeSO_4 \cdot 7H_2O}

Marking Scheme

  • 1Identification of the crystalline formula for each compound. (0.5 marks)
  • 2Comparison of the number of water molecules and selection of the maximum. (0.5 marks)

Hint

Recall the chemical names and formulas for common hydrates like green vitriol (7), blue vitriol (5), gypsum (2), and washing soda (10).

Quick Oral Answer

Sodium carbonate in its crystalline form is known as washing soda, and its formula is Na2CO3⋅10H2O\mathrm{Na_2CO_3 \cdot 10H_2O}. It contains 10 molecules of water of crystallisation, which is more than ferrous sulphate (7), copper sulphate (5), or calcium sulphate (2).

Analysis & Explanation

Water of crystallisation is the fixed number of water molecules present in one formula unit of a salt. Based on the chemical formulas of the crystalline forms of these compounds:

  1. Ferrous sulphate: As mentioned in Activity 1.5 of the text, its crystalline form is FeSO4⋅7H2O\mathrm{FeSO_4 \cdot 7H_2O}, containing 7 molecules of water.
  2. Copper sulphate: Its crystalline form is CuSO4⋅5H2O\mathrm{CuSO_4 \cdot 5H_2O}, containing 5 molecules of water.
  3. Calcium sulphate: Its crystalline form (Gypsum) is CaSO4⋅2H2O\mathrm{CaSO_4 \cdot 2H_2O}, containing 2 molecules of water.
  4. Sodium carbonate: Its crystalline form (Washing soda) is Na2CO3⋅10H2O\mathrm{Na_2CO_3 \cdot 10H_2O}, containing 10 molecules of water.

Comparing these, Na2CO3\mathrm{Na_2CO_3} has the maximum number (10) of water molecules per formula unit.

Common Mistakes

  1. 1Confusing the 7 molecules of water in Ferrous sulphate with the 5 molecules in Copper sulphate.
  2. 2Forgetting that the 'crystalline form' implies the hydrated state of the salt.

Interesting Facts

The text notes that FeSO4⋅7H2O\mathrm{FeSO_4 \cdot 7H_2O} loses its green colour when heated because the water of crystallisation is driven off, changing its chemical structure before it decomposes into ferric oxide.

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2024?

This question carries 1 mark in the CBSE Class 10 Science 2024 examination.

Which chapter does this question come from in Science?

This question is from the chapter "Chemical Reactions and Equations" in the CBSE Class 10 Science syllabus.

What topic does this question cover in Science?

This question covers the topic "Decomposition Reaction" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2024 paper?

This is a MCQ question from Section A in the CBSE Class 10 Science 2024 paper.