Question

A recipe for an ointment known as "psoriasis ointment" reads as follows:

Coal Tar Solution 100 parts

Zinc Oxide Powder 200 parts

Hydrophilic Ointment 1000 parts

A prescription calls for the pharmacist to dispense 30 g of "psoriasis ointment".

How many milliliters of Coal Tar Solution (S.G. 0.84) must the pharmacist measure in order to dispense 30 g of the ointment?

The answer is 2.7, but how is this calculated?

Answer 1

ointment composition is:

coal tar solution: Zinc oxide powder: hydrophilic ointment

100 parts: 200 parts: 1000 parts

So the ratio will be

100:200:1000 or

1:2:10

Now, 1 + 2 + 10 = 13

So in 13 parts, 1 part is coal tar

Therefore for 30 gram of ointment, we can calculate the weight of the coal tar as follows:

1/13 = x/30

13x = 30

x = 30/13

x = 2.3

Now, the surface gravity of coal tar is 0.84,

therefore the number milliliters required for 30 g of ointment will be

2.3 x 0.84 = 2.73 or 2.7 mL approx

So the volume of coal tar solution required is 2.7 mL