68. Solving a chemistry problem

68. Solving a chemistry problem

12. Table 4 shows examples of titrations. Determine which method is used in each specific case: a) direct titration; b) back titration; c) substitution titration. Give the formula for calculating the results of the analysis.
Table 4
Option Substance Reaction Equation Titration
12 Pb2 Pb2 CrO42- ↔ PbCrO4(T)
2 PbCrO4(T) 2H ↔ 2Pb2 Cr2O72- H2O
Cr2O72- 6I- 14H ↔3I2 2Cr3 7H2O
I2 2S2O32- ↔ 2J- S4O62- The released iodine I2 is titrated with a solution of Na2S2O3

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