Section 2: Calculating the Mass of a Component


If you know the % w/w of a component and the mass of product involved, you can calculate the mass of the component in that volume.





How many grams of Zinc Oxide are there in 240 grams of Zinc Ointment BP 1988 containing 15 % w/w Zinc Oxide?

           
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Ratio Solution
A concentration of 15 % w/w Zinc Oxide means that:
100 g of ointment contains 15 g of Zinc Oxide.

Thus:
240 g of ointment contains 240 x 15 g of Zinc Oxide
100

240 g of ointment contains 36 g of Zinc Oxide

Equation Solution
The equation you can use is:
% w/w = mass of solute x             100                      (g/100 g)
mass of product
which rearranges to:
mass of solute = % w/w x mass of product   g
100
mass of Zinc Oxide = 15 x 240 g
100
mass of Zinc Oxide= 36 g
240 grams of Zinc Ointment contains 36 grams of Zinc Oxide.