UPSC Combined Geo-Scientist, Mains · 2024 · Mains Paper I
Q1(d). Silica by volatilisation gravimetry
Question
A sample of slag from a blast furnace is analyzed for SiO₂ by decomposing a 0.5003 g sample with HCl, leaving a residue with a mass of 0.1414 g. After treating with HF and H₂SO₄, and evaporating the volatile SiF₄, a residue with a mass of 0.0183 g remains. Determine the % w/w SiO₂ in the sample.
Answer in brief
Mass of SiO₂ = 0.1231 g; SiO₂ = 24.61% w/w.
Key points
- The first residue contains silica and nonvolatile impurities. HF removes the silica as volatile silicon fluoride.
- Reaction: SiO₂ + 4HF → SiF₄↑ + 2H₂O.
- Mass by difference: m(SiO₂) = 0.1414 − 0.0183 = 0.1231 g.
- Percentage: (0.1231/0.5003) × 100 = 24.61% w/w. The final residue corrects for material that was not silica.
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.
Coversvolatilisation gravimetrysilica determinationSiF4HF treatmentblast furnace slag
All parts of Question 1
- 1(a) Minerals and their compositions5 marks
- 1(b) Magnetic moment of titanium(III)5 marks
- 1(c) Choosing indicators5 marks
- 1(d) Silica by volatilisation gravimetry5 marks
- 1(e) Spontaneity of Fe²⁺ oxidation5 marks
- 1(f) Oxidising power and charge transfer5 marks
- 1(g) pH dependence of cyanide masking5 marks
- 1(h) Terminal and bridging carbonyl bands5 marks
- 1(i) Identifying radioactive decay5 marks
- 1(j) Why ordinary lanthanoid carbonyls are unstable5 marks