Materials Vitamin C tablets · 0.1 M NaOH (standardized) 50 mL buret . buret funn

ID: 905981 • Letter: M

Question

Materials Vitamin C tablets · 0.1 M NaOH (standardized) 50 mL buret . buret funnel centigram balance ring stand& buret clamp ing stand & 100 mL graduated cylinder weigh boat . phenolphthalein . vinegar . mortar & pestle 250 mL beaker powder funnel . marker 250 mL Erlenmeyer flask Procedure Part A. Preparing and Filling the Buret Obtain approximately 75 mL of standardized NaOH in a graduated cylinder. The exact measurement is not important! Obtain a 50 mL buret, a buret clamp, and a ring stand. Mount the buret on the stand as instructed. Rinse and fill the buret using the following procedure: Close the stopcock. Using the buret funnel, add 5.10 mL of NaOH to the buret. Remove the funnel and then remove the buret from the stand. Holding the buret almost horizontally, rotate it so that the NaOH solution contacts the entire inner surface. . Clamp the buret to the stand once again. Open the stopcock and allow the NaOH solution to drain into your waste beaker. Close the stopcock. Using the buret funnel, carefully fill the buret with NaOH solution. Add NaOH slowly until it is just above the 0.00 mL line. Remove the funnel and be sure all air bubbles are removed from the buret tip. If bubbles are in the tip, remove them by opening the stopcock and allowing a small amount of NaOH solution to flow into a waste beaker. You may need to do this a few times. Continue draining sufficient NaOH solution so that the bottom of the meniscus is below · . the 0.00 mL calibration mark. The meniscus should not be at 0.00 exactly. . Read and record the initial buret volume to the nearest 0.01 mL. Part B. Preparation of Vitamin C Samples Obtain two Vitamin C tablets, and determine the mass of the pair. Then use a mortar and pestle to crush the tablets into powder. Obtain a clean, dry 250 mL Erlenmeyer flask. Determine the mass of the empty flask, then transfer roughly one-third of the ground Vitamin C powder. Re- mass the flask containing the powder. Part C. Titrating the Vitamin C Samples Add approximately 50 ml distilled water to the flask not be concerned if all of the powder does not dissolve. Re binders and fillers, which may not be soluble in water.) containing Vitamin C powder. (Note: Do member, the tablets contain various 41

Explanation / Answer

Ascorbic acid is monoprotic.

HC6H7O6 + NaOH = NaC6H7O6 + H2O

Its molar mass is M = 176.1 g/mol

Mass sample , g

0.24

0.58

0.41

Volume (mL) of NaOH 0.1 M used for titration

10.52

25.73

18.8

Mass of vit.C in sample, g

0.1852

0.4528

0.3311

Vit C in sample as Mass %

Mass of vit.C in sample (g) is calculated using the formula:

VNaOH (L) x NNaOH (val/L) x M g/val =

…. = 0.01052 L x 0.1 val/L x 176.1 g/val = 0.1852 g acid ascorbic in sample 1

….. = 0.02573 L x 0.1 val/L x 176.1 g/val =0.4528 g acid ascorbic in sample 2

……. = 0.01880 L x 0.1 val/L x 176.1 g/val = 0.3311 g acid ascorbic in sample 3

Mass percent

100 x 0.1852/0.24 = 77% in sample 1 and similar for samples 2, 3.