In what proportion should a 20% zinc oxide ointment be mixed with white petrolatum (diluent) to produce a 3% zinc oxide ointment?

3:17 (ointement:diluent)

Explanation:

Let's assume the production of 1L = 1,000 mL of the 3% zinc oxide ointment. A sample of the 20% ointment must be taken and then diluted, and so the concentration will change, but the mass will remain constant. The mass is the concentration (C) multiplied by the volume (V), thus if 1 is the 20% ointment, and 2 the diluted:

C1*V1 = C2*V2

C1 = 20%

V1 = ?

C2 = 3%

V2 = 1,000 mL

20V1 = 3*1000

V1 = 150 mL

Thus, it's necessary 150 mL of the ointment and 850 mL (1,000 - 150) of the white petrolatum. The proportion is the ratio between the two substances:

150/850 = 15/85 = 3/17

Thus, they will be mixed in the proportion of 3:17 (ointment:diluent).