1. An aspirin tablet was ground to a fine powder and dissolved in 10 ml of sodiu

ID: 698834 • Letter: 1

Question

1. An aspirin tablet was ground to a fine powder and dissolved in 10 ml of sodium hydroxide. This solution was quantitatively transferred to a 250 ml volumetric flask and diluted to the mark with distilled water. An aliquot (5.00 ml) of this solution was transferred to each of five 100 ml volumetric flasks. To each flask a specified volume of 1.015 mg/ml acetylsalicylic acid solution was added. The samples were diluted to the mark with 0.02 M iron chloride solution. The absorbance for each sample was measured at 530 nm and recorded. Volume 1.015 mg/ml acetylsalicylic acid added 0.00 ml 2.50 ml 5.00 ml 7.50 ml 10.0 ml Absorbance 0.145 0.220 0.289 0.363 0.435 How many mg aspirin are in the tablet?

Explanation / Answer

We need to plot the absorbances of the spiked samples against the concentration of acetylsalicylic acid added. Use the dilution equation to figure out the concentrations of added acetylsalicylic acid in the 100 mL volumetric flasks. Take the third solution (5.00 mL of 1.015 mg/mL) standard as an example. We have

M1*V1 = M2*V2

where M1 = 1.015 mg/mL; V1 = 5.00 mL and V2 = 100 mL.

Therefore,

M2 = M1*V1/V2 = (1.015 mg/mL)*(5.00 mL)/(100 mL) = 0.05075 mg/mL.

Prepare the following table.

Volume of 1.015 mg/mL acetylsalicylic acid added (mL)

Concentration of added acetylsalicylic acid in the test solution (mg/mL)

Absorbance of the test solution

0.00

0.0

0.145

2.50

0.025375

0.220

5.00

0.050750

0.289

7.50

0.076125

0.363

10.00

0.101500

0.435

Plot the absorbance against the concentrations of the added standard.

The concentration of acetylsalicylic acid (Vitamin C) in the tablet is obtained from the regression equation. Put y = 0 in the regression equation and obtain

0 = 2.8493x + 0.1458

=====> -2.8493x = 0.1458

=====> x = 0.1458/(-2.8493) = -0.05117

The concentration of Vitamin C cannot be negative; hence, ignore the negative sign and obtain the concentration of Vitamin C in the dilute test solution as 0.05117 mg/mL.

The test solution was prepared by taking 5.00 mL of the prepared stock solution and diluting to 100.0 mL; the dilution factor is (100.0 mL)/(5.00 mL) = 20.0.

The concentration of Vitamin C in the stock solution = (0.05117 mg/mL)*(20.0) = 1.0234 mg/mL.

The stock solution was prepared by dissolving the tablet in 10.0 mL of sodium hydroxide and diluting to 250.0 mL; the dilution factor is (250.0 mL)/(10.0 mL) = 25.0.

The concentration of Vitamin C in the sodium hydroxide extract is (1.0234 mg/mL)*(25.0) = 25.585 mg/mL.

We prepared 10.0 mL of sodium hydroxide extract; hence, the mass of Vitamin C in the tablet is (10.0 mL)*(25.585 mg/mL) = 255.85 mg (ans).