An ointment contains 1% w/w zinc oxide. how much zinc oxide would it be appropriate to add to 400 g of the ointment to produce a 4% w/w zinc oxide ointment?

12 g of Zinc Oxide are needed.

Explanation:

First, let's take a look at what % w/w means. This is a way to represent concentration of a compound, zinc oxide in this case, and it means: grams of zinc oxide, per 100 grams of solution (or ointment in this case). So, for this particular ointment we have 1 g of Zinc Oxide present per 100 g of the ointment.

Now, we want to prepare 400 g of a 4 %w/w ointment from the 1 %w/w ointment and adding solid zinc oxide. Let's see how many grams of zinc oxide would be present in these 400 g of a 4 %w/w ointment:

4 %w/w = 100 g of ointment 4 g of zinc oxide

                400 g of ointment x = (400 g × 4 g)/ 100 g = 16 g of zinc oxide

Our base ointment has only 1 g of zinc oxide per 100 g of product, so in 400 g of product we would have 4 g of compound. Therefore, we would need to add 12 g of zinc oxide (16 g needed - 4 g we already have) to 400 g of base ointment so that, when dissolved, the resulting ointment would have 16 g total of zinc oxide, or what is the same as seen before, 4 % w/w ointment.