Question

1. Calculate the molarity of a solution made by dissolving 0.4000 g of acetylsalicylic acid (180.15 g/mol) in 250.00 mL of water.

2. Calculate the volume of the solution prepared in (1) needed to make 100.00 mL of solution with a concentration of 4.00 x 10–4 M.

3. What are the correct units for molar absorptivity according to Beer’s Law?

4. Describe how you will determine the concentration of acetylsalicylic acid in your individual diluted aspirin-tablet solutions. (Hint: Think Beer’s Law Plot)

5. Set up the calculation for the concentration of acetylsalicylic acid in the undiluted 250-mL aspirin-tablet solution. (Assume a conc. of 3.60 x 10-4 M from the standard curve).

6. Set up the calculation for the mass percent of acetylsalicylic acid in the tablet your team tested. (Use your answer from previous question and use a total mass for the tablet of 0.4000g).

Answer 1

1. Calculate the molarity of a solution made by dissolving 0.4000 g of acetylsalicylic acid (180.15 g/mol) in 250.00 mL of water.

moles of aspirin = mass / molar mass

                            = 0.400 / 180.15

                            = 2.22 x 10^-3

molarity = moles / volume (lit)

             = 2.22 x 10^-3 / 0.250

               = 8.88 x 10^-3 M

2. Calculate the volume of the solution prepared in (1) needed to make 100.00 mL of solution with a concentration of 4.00 x 10–4 M.

C1 = 8.88 x 10^-3 M ,   V1 = ?

C2 = 4.00 x 10^–4 M. , V2 = 100 mL

C1 V1 = C2 V2

8.88 x 10^-3 x V1 = 4 .00 x 10^-4 x 100

V1 = 4.5 mL

volume needed = 4.5 mL

3. What are the correct units for molar absorptivity according to Beer’s Law?

molar absorptivity units = M-1 cm-1

or M-1 mm-1