Formulas Specifying Weight or Volume

Factor method can be used to reduce or enlarge formulas that express metric quantities in weight or volume.

Solved Problem: The following formula is for Nitrofurazone 0.02 % Nasal Solution/Spray. Calculate the quantity of each ingredient to prepare 250 ml of product.

Ingredient
Nitrofurazone 20 mg
Phenylephrine HCl 250 mg
Polyethylene glycol 300 10 ml
Sodium chloride 800 mg
Bacteriostatic water qs 100 ml

Approach: Use the equation to determine the factor. Multiply each ingredient with the factor.

$$Factor = {250 \ ml (desired) \over 100 \ ml (formula \ given) } = 2.5 $$

Using the factor 2.5, quantity of each ingredient can be determine.

Nitrofurazone 20 mg X 2.5 = 50 mg
Phenylephrine HCl 250 mg X 2.5 = 625 mg
Polyethylene glycol 300 10 ml X 2.5 = 25 ml
Sodium chloride 800 mg X 2.5 = 2000 mg
Bacteriostatic water qs 100 ml X 2.5 = 250 ml

Answer: Nitrofurazone 50 mg; Phenyephrine HCl 625 mg; Polyethylene glycol 300 25 ml; Sodium chloride 2000 mg; bacteriostatic water qs 250 ml.