Assay of Zinc Oxide 

Molecular Formula: ZnO 

Molecular Weight: 81.38 

Assay: Weigh accurately about 1.5 g of zinc oxide and dissolve with 2.5 g of ammonium chloride in 5 mL of 1N H₂SO₄ with the aid of gentle heat, if necessary. On complete dissolution, add a few drops of methyl orange solution and titrate the excess sulphuric acid with 1N NaOH. The endpoint will be red to yellow. Carry out blank determination omitting zinc oxide sample. The difference in the two titrations indicates the amount of 1N H₂SO₄ reacting with zinc oxide. 

Reactions: 

ZnO  + H₂SO₄ → ZnSO₄ + H₂O + H₂SO₄ (Excess) 

H₂SO₄ +  2NaOH → Na₂SO₄ + 2H2O  (Factor)

Calculation: 

1 Mole of H₂SO₄ ≅ 1 Mole of zinc oxide 

1000 mL of 1M H₂SO₄ ≅ 81.38 g of ZnO 

1 mL of 1N H₂SO₄ ≅ 0.04069 g of ZnO

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