Which of the following is 'a' FALSE statement?

Step 1: Identify which statement is FALSE

(a) "Carius method is used for the estimation of nitrogen in an organic compound."

❌ FALSE — The Carius method is used for the estimation of halogens and sulphur, NOT nitrogen. For nitrogen, the methods used are Dumas' method and Kjeldahl's method. Statement (a) is the false statement.

(b) "Carius tube is used in the estimation of sulphur in an organic compound."

✅ TRUE — The Carius method uses a thick-walled, sealed glass tube called a Carius tube. This tube is used for both halogen and sulphur estimation.

(c) "Kjeldahl's method is used for the estimation of nitrogen in an organic compound."

✅ TRUE — Kjeldahl's method is indeed used for nitrogen estimation. The compound is heated with conc. H₂SO₄ → $\ce{(NH4)2SO4}$ → $\ce{NH3}$ liberated → absorbed in acid → titrated.

(d) "Phosphoric acid produced on oxidation of phosphorus is precipitated as $\ce{Mg2P2O7}$ by adding magnesia mixture."

✅ TRUE — $\ce{H3PO4}$ + magnesia mixture ($\ce{MgCl2}$ + $\ce{NH4Cl}$ + $\ce{NH3}$) → $\ce{MgNH4PO4}$ (white ppt) → ignited → $\ce{Mg2P2O7}$ (magnesium pyrophosphate). This is the standard gravimetric method for phosphorus estimation.

Step 2: Conclusion

The FALSE statement is (a) — Carius method is for halogens/sulphur, NOT nitrogen.

Answer: (a)

Key Points to Remember:

• Nitrogen estimation: Dumas' method + Kjeldahl's method
• Halogen estimation: Carius method (AgX precipitate)
• Sulphur estimation: Carius method (BaSO₄ precipitate)
• Phosphorus estimation: Oxidise → $\ce{H3PO4}$ → magnesia mixture → $\ce{Mg2P2O7}$