If 60 mL of a combination gel of hydrocortisone acetate, 1% w/w, and pramoxine, 1% w/w, is mixed with 12.5 mL of a gel containing hydrocortisone acetate, 2.5% w/w, and pramoxine, 1% w/w,

Question

calculate the percentage strength of each of the

two drugs in the mixture

Applying this: Altering Strength, Stock Solutions, and Alligation Calculations

Answer 1

Find mass of each active ingredient. Find total mass of the preparation. Divide to get % w/w.

Unfortunately we don' t know the density of these solutions, so since we are given 60 mls (volume) and1% w/w/ (mass) we can't really solve this unless we assume the density is 1 g/ml. Maybe they meant the % was w/v and not w/w. If we make the density assumption, we can proceed as follows:

60 ml x 1 g / ml = 60 g x 1% pramoxine = 0.6 g pramoxine

60 ml x 1 g / ml = 60 g x 1% hydrocort = 0.6 g hydrocort

12.5 ml x 1 g / ml = 12.5 g x 1% pramoxine = 0.125 g pramoxine

12.5 ml x 1 g / ml = 12.5 g x 2.5% = 0.3125 g hydrocort

total mass pramoxine = 0.6 g + 0.125 g = 0.725 g

total mass hydrocort = 0.6 g + 0.3125 g = 0.9125 g

total mass of preparation = 60 g + 60 g + 12.5 g + 12.5 g = 145 g

% strength pramoxine = 0.725 g / 145 g (x100%) = 0.50%

% strength hydrocortisone = 0.9125 g / 145 g (x100%) = 0.63%