When 3.17 g of mercury thiocyanate [Hg(SCN)2] is heated in a well-ventilated fume cupboard it decomposes completely according to the following balanced equation - Leaving Cert Chemistry - Question c - 2019

Balanced Reaction for CS₂ Combustion:

• $\text{CS}_2 + 3\text{O}_2 \rightarrow \text{CO}_2 + 2\text{SO}_2$

Moles of CS₂ Produced:

• From the decomposition reaction, 1 mole of CS₂ is produced from 0.0100 moles of Hg(SCN)₂, hence 0.00500 moles of CS₂ is produced.

Calculate Required Moles of O₂:

• For every 1 mole of CS₂, 3 moles of O₂ are required (from the balanced equation): 0.00500 moles CS₂ requires:
• O₂ required = 0.00500 moles × 3 = 0.0150 moles O₂

Convert Moles of O₂ to Litres at STP:

• At STP, 1 mole of gas occupies 22.4 litres.
• Therefore, O₂ required in litres = 0.0150 moles × 22.4 L/mol = 0.336 litres.

Moles of Hg Produced from Reaction:

• From the balanced equation, for every 2 moles of Hg(SCN)₂, 2 moles of Hg are produced. Since we have 0.0100 moles of Hg(SCN)₂:
• Moles of Hg produced = 0.0100 moles.

Calculate Number of Atoms of Hg:

• Number of atoms = Moles × Avogadro's number (6.022 × 10²³ atoms/mol):
• Number of Hg atoms = 0.0100 moles × 6.022 × 10²³ atoms/mol = 6.02 × 10²¹ atoms.

Identify All Gaseous Products:

• From the previous equations:
  • 0.0100 moles HgO forms gaseous products when decomposed to form 0.0100 moles SO₂, which does not contribute to the gas count.
  • 0.00500 moles CS₂ requires 0.0150 moles O₂ for combustion, and produces gaseous products:
    • CO₂ (1 mole) and SO₂ (2 moles): 3 moles from combustion of CS₂.
  • 0.010 moles of CN (in the decomposition reaction) forms 0.00500 moles N₂.

Calculate Total Moles of Gases:

• Total moles of gaseous products = 0.010 moles SO₂ + 0.0150 moles O₂ + 0.00500 moles N₂ = 0.035 moles.