The amount of Cr3+ in an inorganic salt can be determined by a redox back titration. A portion of sample containing approximately 0.25 g of Cr3+ is accurately weighed and dissolved in 50 mL of H2O. The Cr3+ is oxidized to Cr2O72- by adding 20 mL of 0.1 M AgNO3, which serves as a catalyst, and 50 mL of 10%w/v (NH4)2S2O8, which serves as the oxidizing agent (reaction 1). After the reaction is complete the resulting solution is boiled for 20 minutes to destroy the excess S2O82-, cooled to room temperature, and diluted to 250.0 mL in a volumetric flask. A 50-mL portion of the resulting solution is transferred to an Erlenmeyer flask, treated with 50 mL of a standard solution of Fe2+, and acidified with 200 mL of 1 M H2SO4, reducing the Cr2O72- to Cr3+ (reaction 2). The excess Fe2+ is then determined by a back titration with a standard solution of K2Cr2O7 (reaction 2 again) using an appropriate indicator. The results are reported as %w/w Cr3+.
reaction 1: 2 Cr3+ + 7 H2O â Cr2O72- + 14 H+ + 6 e- S4O82- + 2 e- â 2 SO42-
reaction 2: Fe2+ â Fe3+ + e- Cr2O72- + 14 H+ + 6 e- â 2 Cr3+ + 7 H2O
There are several places in the procedure where a reagents volume is specified (see underlined text). Which of these measurements must be made using a volumetric pipet? Select all that apply.
50 mL of H2O
20 mL of 0.1 M AgNO3
50 mL of 10%w/v (NH4)2S2O8
50-mL portion of the resulting solution
50 mL of a standard solution of Fe2+
200 mL of 1 M H2SO4
The amount of Cr3+ in an inorganic salt can be determined by a redox back titration. A portion of sample containing approximately 0.25 g of Cr3+ is accurately weighed and dissolved in 50 mL of H2O. The Cr3+ is oxidized to Cr2O72- by adding 20 mL of 0.1 M AgNO3, which serves as a catalyst, and 50 mL of 10%w/v (NH4)2S2O8, which serves as the oxidizing agent (reaction 1). After the reaction is complete the resulting solution is boiled for 20 minutes to destroy the excess S2O82-, cooled to room temperature, and diluted to 250.0 mL in a volumetric flask. A 50-mL portion of the resulting solution is transferred to an Erlenmeyer flask, treated with 50 mL of a standard solution of Fe2+, and acidified with 200 mL of 1 M H2SO4, reducing the Cr2O72- to Cr3+ (reaction 2). The excess Fe2+ is then determined by a back titration with a standard solution of K2Cr2O7 (reaction 2 again) using an appropriate indicator. The results are reported as %w/w Cr3+.
reaction 1: 2 Cr3+ + 7 H2O â Cr2O72- + 14 H+ + 6 e- S4O82- + 2 e- â 2 SO42-
reaction 2: Fe2+ â Fe3+ + e- Cr2O72- + 14 H+ + 6 e- â 2 Cr3+ + 7 H2O
There are several places in the procedure where a reagents volume is specified (see underlined text). Which of these measurements must be made using a volumetric pipet? Select all that apply.
50 mL of H2O
20 mL of 0.1 M AgNO3
50 mL of 10%w/v (NH4)2S2O8
50-mL portion of the resulting solution
50 mL of a standard solution of Fe2+
200 mL of 1 M H2SO4
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