I. Sample Experiment 2.5 points A student takes a 0.2501 g pain-reliever tablet

Question

I. Sample Experiment 2.5 points A student takes a 0.2501 g pain-reliever tablet and dissolves it in a 250 mL volumetric flask. A 2.0 mL aliquot of this clear solution is taken and diluted to 50.00 mL with iron (III) chloride. The absorbance of the resulting purple solution was measured at 530 nm. From the calibration curve, the concentration of the iron(II) salicylate complex in the sample was equal to 1.2 x 10-4 M. Given this information, please answer the following questions. For full credit, show all of your calculations on this page. 1. How many moles of the Fe(II) salicylate complex (the chromophore) were present in the 50 mL volumetric flask? mol Fe(III) salicylate 2. What is the molarity of the salicylate in the 250 ml volumetric flask (Hint: is the 2mL aliquot important?) M salicylate 3. What is the number of moles of salicylate in the 250 mL volumetric flask? mol salicylate 4. What is the number of grams of acetylsalicylic acid in the pain-reliever tablet? g acetylsalicylic acid

Explanation / Answer

1. concentration = 1.2*10-4 M, so mole = 50/1000*1.2*10-4 = 6*10-6 mole

2. so 2 ml aliquote contain 6*10-6 mole, so molarity = 6*10-6 *1000/2 = 3*10-3 (M) so molarity of 250 ml volumetric flask = 3*10-3 (M)

3. no of mole in 250 ml = 3*10-3 (M)*250/1000 = 7.5*10-4

4. no of gram = molecular weight * 7.5*10-4 = 180.157*7.5*10-4 = 0.135 g

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