A 1.85 ~g sample of an arsenic-containing pesticide was chemically converted to AsO ₄ ³⁻ (atomic mass of As = 74.9) and titrated with Pb ²⁺ to form Pb ₃ ( AsO ₄ )₂ If 20 ~mL of 0.1 M Pb ²⁺ is required to reach the equivalence point, the mas

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A 1.85 ~g sample of an arsenic-containing pesticide was chemically converted to AsO ₄ ³⁻ (atomic mass of As = 74.9) and titrated with Pb ²⁺ to form Pb ₃ ( AsO ₄ )₂ If 20 ~mL of 0.1 M Pb ²⁺ is required to reach the equivalence point, the mass percentages of arsenic in the pesticide sample is closest to

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Correct answer: C. 5.4

3 pb ²⁺+2 ASO ₄³⁻ pb ₃ ( AsO ₄ )₂ n = M V n = 2 3 2 10⁻³=0.1 20 1000 =0.00133=2 10⁻³ _ AS = _ ASO ₄³ =0.00133 ~W _ AS =0.00133 74.9=0.0996 % of AS = 0.0996 1.85 100=5.4 %

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