A pharmacist is to prepare 300 g of an ointment containing 50 mL of glycerin. The density of glycerin is 1.25 g/mL.

Question

What is the % w/w of this preparation?

Answer 1

If you have the density of glycerin and the amount of volume of glycerin, you can find out how many grams this would corresponds to:. Then you would see what this would mean for the 300g of the ointment:

1.25g/ml. = Xg/50ml. Grams would equal:. 62.5g.

As you would have 62.5 g of the glycerin by volume according to it's density. But you have been told that you have 300 grams of the ointment.

If you take 300/62.5 = 4.8. So you could make room for 4.8 times the amount of glycerin for the solution.

Or if you did 62.5/300 = 0.2083 or 21% of the solution ointment by mass is the glycerin

Answer 2

50 ml glycerin x 1.25 g / ml = 62.5 g glycerin

% w/w of glycerin = 62.5 g / 300 g (x100%) = 20.83% (not corrected for sig. figs.)

If you correct for sig.figs. the answer would be 20% since 300 g has only 1 sig. figs.