PRE-LAB EXERCISES (answer in the notebook) 1) Calculate the molar concentration

Question

PRE-LAB EXERCISES (answer in the notebook) 1) Calculate the molar concentration (molarity) of a 250 ml stock solution prepared 2) Calculate the molar concentration (molarity) of a 50 mL. solution prepared from 2.5 3) Similarly, calculate the molar concentrations (molarities) of 50 mL the four individual from 0.5 g of pure acetylsalicylic acid. mL of the acetylsalicylic acid stock solution from question 1. This will be labeled as Standard Solution A solutions prepared from 2, 1.5, 1, and 0.5 mL, respectively, of the acetylsalicylic acid stock solution from question 1. These will be labeled as Standard Solutions B, C, D, E, respectively. Transfer the values of molar concentrations obtained into the following table. The absorbance values obtained for the solutions A-E are included. 4) Solution Molar Concentration 0.4846 0.3794 0.2690 0.1913 0.0996 By hand or using Excel, build a standard calibration curve and find the equation of this curve. 5) Briefly explain the purpose of building a standard calibration curve. 6) A spectrophotometric analysis of 0.2223 g aspirin product was performed using the procedure above (Part Ill on page 3). The absorbance of the unknown sample solution of this product was determined to be 0.3574. Use the equation of the standard calibration curve from question 4 and back calculate the amount of pure acetylsalicylic acid present in this aspirin product a. b. Find the % purity of the aspirin product. Not for grading

Explanation / Answer

1. molarity of acetylsalicylic acid solution = grams/molar mass x volume

= 0.5 g/180.157 g/mol x 0.250 L

= 0.011 M [stock solution]

2. 2.5 ml of stock diluted to 50 ml,

molarity of solution A = 0.011 M x 2.5 ml/50 ml = 5.5 x 10^-4 M

3. molarity of solution B = 0.011 M x 2.0 ml/50 ml = 4.4 x 10^-4 M

molarity of solution C = 0.011 M x 1.5 ml/50 ml = 3.3 x 10^-4 M

molarity of solution D = 0.011 M x 1.0 ml/50 ml = 2.2 x 10^-4 M

molarity of solution E = 0.011 M x 0.5 ml/50 ml = 1.1 x 10^-4 M

Equation for the curve

y = -0.0958x + 0.5702

4. Standard curve below

5. A calibration plot or standard curve is plotted in a way that any unknown solution concentration can than be calculated from it.

6. A sample of aspirin was analyzed

a. amount of pure acetylsalicylic acid = [(0.3574-0.5722)/-0.0958]0.9960 = 2.23 x 10^-4 M

b. % purity of aspirin = 2.23 x 10^-4 M x 0.05 L x 180.157 g/mol x 100/0.2223 g = 0.904%

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