Section 2: Calculating the Mass of a Component
If you know the % w/v of a component and the volume of product involved, you can calculate the mass of the component in that volume.
How many grams of Coal Tar are there in 250 ml of Coal Tar and Salicylic Acid Ointment BP 1988 containing 2 % w/v Coal Tar?
Click to Remove Question
Ratio Solution
There are 5 g of Coal Tar in 250 ml of Ointment.
A concentration of 2 % w/v Coal Tar means that:
100 ml of ointment contains 2 g of Coal Tar
Thus: 250 ml of ointment contains 250 x 2 g of Coal Tar
100
100
250 ml of ointment contains 5 g of Coal Tar
Equation Solution
There are 5 g of Coal Tar in 250 ml of Ointment.
% w/v = mass of solute x 100 (g/100ml)
volume of product
volume of product
which rearranges to:
mass of solute = % w/v x volume of product g
100
100
mass of Coal Tar = 2 x 250 g
100
mass of Coal Tar = 5 g
100
mass of Coal Tar = 5 g