UPSC Combined Geo-Scientist, Mains · 2024 · Mains Paper III
Q1(d). Identifying a magnesium compound from its thermogram
Question
A thermogram of a magnesium compound showed a loss of 66.0 mg from a total of 126.5 mg used for the analysis. Identify the compound either as MgO or MgCO₃ or MgC₂O₄. [Mg = 24.3, C = 12, O = 16]
Answer in brief
Loss found = 52.2 %. Only MgCO₃ → MgO + CO₂ (calculated loss 52.2 %) fits; the compound is MgCO₃.
Key points
- Observed loss: 66.0/126.5 × 100 = 52.2 %; residue 60.5 mg, the final residue of each compound being MgO.
- MgO (M = 40.3): thermally stable, no loss (0 %).
- MgCO₃ (M = 84.3) → MgO + CO₂ (44): loss = 52.2 %, that is 66.0 mg from 126.5 mg. Matches.
- MgC₂O₄ (M = 112.3) → MgO + CO + CO₂ (72): loss = 64.1 %, that is 81.1 mg. Does not match.
- Mole check: 126.5 mg = 1.50 mmol MgCO₃; it leaves 1.50 mmol MgO = 60.5 mg, as found.
The full model answer
As written for the answer booklet: the diagram carries the working, the lines stay short. Read here only; it is not offered as a download.
CoversthermogravimetrythermogramMgCO3magnesium oxalatedecompositionmass loss
All parts of Question 1
- 1(a) Percentage relative error of a balance reading5 marks
- 1(b) Partition ratio k′ and relative retention α5 marks
- 1(c) Transmittance from Beer’s law5 marks
- 1(d) Identifying a magnesium compound from its thermogram5 marks
- 1(e) Properties needed in a detector for X-ray fluorescence5 marks
- 1(f) Generation of the plasma in the ICP torch5 marks
- 1(g) Calorific value of a fuel5 marks
- 1(h) AAS and ICP compared: accuracy, interferences, detection limit5 marks