Combustion Analysis of H₂S and CS₂

H₂S and CS₂ are present in 3 to 1 ratio, hence volumes of H₂S and CS₂ are 30 cm³ and 10 cm³ respectively.

Question mentioned any sulfur present is converted to SO₂ after combustion, and we will expect hydrogen to be converted to H₂O and carbon converted to CO₂ after combustion.

Then we can balanced the equations for combustion of H₂S and CS₂.

Comparing mole ratio of gases we can deduce 30 cm³ of H₂S reacts with 45 cm³ of O₂ to form 30 cm³ of SO₂.

We can ignore water produced since it is liquid at room temperature and does not contribute to volume of gases.

10 cm³ CS₂ reacts with 30 cm³ O₂ to form 10 cm³ CO₂ and 20 cm³ SO₂.

Now we can put everything together:

Total volume of O₂ reacted = 45 + 30 = 75 cm³
Vol of O₂ remaining = 100 - 75 = 25 cm³

Vol of CO₂ formed = 10 cm³
Vol of SO₂ formed = 30 + 20 = 50 cm³

Hence,

Vol of gases after burning = vol of O₂ remaining + vol of CO₂ + vol of SO₂ = 25 + 10 + 50 = 85 cm³

After treated with excess NaOH(aq), acidic gases CO₂ and SO₂ will be absorbed.

Final vol after adding NaOH(aq) = 25 cm³

Therefore answer to this question is option C.

Topic: Mole Concept, Physical Chemistry, A Level Chemistry, Singapore

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