Question

A standard solution is prepared for the analysis of fluoxymesterone (C20H29FO3), an anabolic steroid. A stock
solution is first prepared by dissolving 10.0 mg of fluoxymesterone in enough water to give a total volume of
500.0 ml. A 100.0 μL aliquot (portion) of this solution is diluted to a final volume of 100.0 ml. Calculate the
concentration (2 sig. figs.) of the final solution in terms of molarity.

Answer 1

Step 1 — molar mass:
C20H29FO3 = 20(12.011) + 29(1.008) + 18.998 + 3(15.999) = 336.45 g·mol^-1

Step 2 — moles in 10.0 mg:
n = 0.0100 g / 336.45 g·mol^-1 = 2.97 × 10^-5 mol

Step 3 — stock concentration (10.0 mg in 500.0 mL = 0.5000 L):
Cstock = 2.97 × 10^-5 mol / 0.5000 L = 5.94 × 10^-5 M

Step 4 — dilution (100.0 μL = 0.1000 mL diluted to 100.0 mL → factor = 0.1000/100.0 = 1.00 × 10^-3):
Cfinal = Cstock × 1.00 × 10^-3 = 5.94 × 10^-8 M

Rounded to 2 significant figures: 5.9 × 10^-8 M.