Question

The molar absorptivity constant for the allergy drug Zyrtec (cetirizine) (MW 388.89 g/mol) is 7,250 L/mol*cm at 245nm. Given a stock solution with a cetirizine concentration of 0.537 mg/mL, specifically describe the best way to prepare a standard solution that would give you an absorbance of about 0.6 @ 245nm? What is the cetirizine concentration of that standard solution?

Answer 1

Answer 1

Concentration of stock = 0.537 mg/ml

= 0.537 g/L

Molar mass of cetrizine = 388.89 g/mol

Concentration of stock (in mol/L) = (0.537 g/L)/(388.89 g/mol)

= 0.001381 mol/L

Molar absorptivity constant (E) = 7250 L/mol cm

Absorbance (A) = 0.6

A = E*C*l

Assuming l (path length) = 1 cm for a spectroscope,

0.6 = 7250 L/mol cm * C * 1 cm

C = 0.00008275 mol/L

Concentration of required cetrizine standard = 0.00008275 mol/L

Volume of stock to be added to form 10 ml of required cetrizine standard :

= (0.00008275 mol/L * 10 ml)/(0.001381 mol/L)

= 0.599 ml

= 0.6 ml

Hence, 0.6 ml of stock must be taken and final volume should be made up to 10 ml with solvent.

Answer 2

Moles of Fe(NO₃)₃.9H₂O = (50.50 g) / (403.999 g/mol)

= 0.125 mol

Volume of solution = 250 ml = 0.250 L

Molarity of solution = 0.125 mol / 0.250 L = 0.5 M

Molarity of Fe³⁺ in stock = 0.5 M

Molarity of required solution of Fe³⁺ = 0.010 M

Let the volume of required solution to be made = 10 ml

So, volume of stock to be added = (0.010 M)*10 ml/ 0.5 M

= 0.2 ml

Hence, take 0.2 ml of stock and make final volume up to 10 ml with solvent.