2. A sample containing Fe2 ions was dissolved in a 200 ml volumetric flask. 10.0 ml of Knopp's mixture and 3 drops of diphenylamine were added to an aliquot of 20.0 ml. 12.2, 12.1, 12.4, 12.1 ml of 0.01 M K2Cr2O7 solution were used for titration by pipetting until a violet color was obtained. Determine the mass of iron in the sample, the standard deviation for the measurement results, the relative standard deviation, and the confidence interval for the mean.

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