Q4(b). Composition of a Ca(OH)₂ sample from its TGA
In the thermogravimetric analysis (TGA) of 0.45 g of Ca(OH)₂ sample, the losses in weight at different temperatures are as follows: (i) 0.025 g at 100°C–150°C; (ii) 0.040 g at 500°C–560°C; (iii) 0.023 g at 900°C–950°C. What is the composition of calcium hydroxide in the sample?
Answer in brief
Ca(OH)₂ =36.6 % (0.1645 g); CaCO₃ =11.6 % (0.0523 g); moisture =5.6 % (0.025 g); the rest, 46.3 % (0.208 g), is CaO and non-volatile matter.
Key points
- n(H₂O)=(0.040)/(18.015)=2.220 mmol =n(Ca(OH)₂). Mass =2.220×74.09=0.1645 g: (0.1645)/(0.45)×100=36.6 %
- n(CO₂)=(0.023)/(44.01)=0.5226 mmol =n(CaCO₃); mass =0.5226×100.09=0.0523 g, i.e. 11.6 %. (The sample has taken up CO₂ from air.)
- Moisture: 0.025/0.45=5.6 %.
- 0.45-0.1645-0.0523-0.025=0.208 g (46.3 %): free CaO and other non-volatile matter (mineral impurities). Residue at 950°C: 0.45-0.088=0.362 g (CaO from the two decompositions =0.1245+0.0293 g, plus 0.208 g).
The full model answer
Worked out step by step, as it should go in the answer booklet. The solution sheet follows it.
| Step | Loss | Process |
| 100–150°C | 0.025 g | loss of adsorbed H₂O (moisture) |
| 500–560°C | 0.040 g | Ca(OH)₂ → CaO + H₂O ↑ (dehydroxylation) |
| 900–950°C | 0.023 g | CaCO₃ → CaO + CO₂ ↑ (decarbonation) |
- Ca(OH)₂: n(H₂O)=(0.040)/(18.015)=2.220 mmol =n(Ca(OH)₂). Mass =2.220×74.09=0.1645 g: (0.1645)/(0.45)×100=36.6 %
- CaCO₃: n(CO₂)=(0.023)/(44.01)=0.5226 mmol =n(CaCO₃); mass =0.5226×100.09=0.0523 g, i.e. 11.6 %. (The sample has taken up CO₂ from air.)
- Moisture: 0.025/0.45=5.6 %.
- Balance: 0.45-0.1645-0.0523-0.025=0.208 g (46.3 %): free CaO and other non-volatile matter (mineral impurities). Residue at 950°C: 0.45-0.088=0.362 g (CaO from the two decompositions =0.1245+0.0293 g, plus 0.208 g).
the same three-step TGA pattern (moisture, hydroxide, carbonate) is used to check the purity and ageing of slaked lime and cement; the CO₂ loss at 900°C is the standard measure of carbonate in limestone and soil.
CoversTGA stoichiometryComposition of a Ca(OH)₂ sample from its TGA
All parts of Question 4
- 4(a) Ion-exchange capacity of an anion-exchange resin10 marks
- 4(b) Composition of a Ca(OH)₂ sample from its TGA10 marks
- 4(c) Gross and net calorific values of coal10 marks