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Materials characterization (SEM, TEM, XRD, XPS)

When calcium oxalate monohydrate (CaC₂O₄·H₂O, molar mass 146.1 g/mol) is heated in TGA, the water of crystallization is released first. Which value is closest to the percent mass loss in this first step? Take the molar mass of H₂O as 18.0 g/mol.

  1. AAbout 12.3%
  2. BAbout 19.2%
  3. CAbout 30.1%
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Answer: A. About 12.3%

The first step is CaC₂O₄·H₂O → CaC₂O₄ + H₂O, losing 18.0 g per mole, so the percent mass loss is 18.0/146.1 × 100 ≈ 12.3%. About 19.2% is the CO (28.0 g/mol) lost in the second step, CaC₂O₄ → CaCO₃ + CO, and about 30.1% is the CO₂ (44.0 g/mol) lost in the third step, CaCO₃ → CaO + CO₂, each divided by the original mass of 146.1 g. Because calcium oxalate monohydrate shows three clearly separated mass-loss steps like this, it is commonly used to check the performance of TGA instruments.

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