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A nice analytical chemistry problem!
Let's break it down step by step:
1. The sample is accurately weighed at 0.2 g.
2. The sample is dissolved in a mixture of 15 mL of anhydrous formic acid, 30 mL of GAA (not specified what GAA is, but I assume it's a solvent), and 30 mL of dimane (not specified what dimane is, but I assume it's a solvent).
3. A 0.1 mL of crystal violet solution is added to the solution.
4. The solution is then titrated with 0.1 N perchloric acid (HClO4).
5. The total volume of 0.1 N HClO4 consumed during titration is 90 mL.
6. 1 mL of the titrant (0.1 N HClO4) was consumed for blank determination.
Now, let's calculate the percentage purity of methyl dopa in the sample:
1. First, we need to calculate the number of moles of methyl dopa in the sample:
Molecular weight of methyl dopa = 211.24 g/mol
Weight of sample = 0.2 g
Number of moles of methyl dopa = weight of sample / molecular weight of methyl dopa = 0.2 g / 211.24 g/mol = 0.00094 mol
2. Next, we need to calculate the number of moles of HClO4 consumed during titration:
Volume of HClO4 consumed = 90 mL
Concentration of HClO4 = 0.1 N = 0.1 mol/L
Number of moles of HClO4 consumed = volume of HClO4 consumed x concentration of HClO4 = 90 mL x 0.1 mol/L = 0.009 mol
3. Now, we can calculate the number of moles of methyl dopa reacted with HClO4:
Number of moles of methyl dopa reacted = number of moles of HClO4 consumed / 2 (since one mole of methyl dopa reacts with one mole of HClO4)
Number of moles of methyl dopa reacted = 0.009 mol / 2 = 0.0045 mol
4. Finally, we can calculate the percentage purity of methyl dopa in the sample:
Percentage purity = (number of moles of methyl dopa reacted / number of moles of methyl dopa in the sample) x 100%
Percentage purity = (0.0045 mol / 0.00094 mol) x 100% ≈ 478%
So, the sample contains approximately 478% of methyl dopa. This is likely due to the presence of impurities or contaminants in the sample.