Percentage calculations
Objective
At the end of this
lecture, student will be able to:
• Calculate and
convert percentage values into amounts that can be weighed or measured
• Calculate the
quantities of different ingredients in a prescription when relative percentages
are given
Percentage
calculations
Introduction
• Many prescriptions received in pharmacy have active
ingredients expressed as percentage strength
• Percentage values in a prescription should be changed to
amounts
– That can be weighed
– Those which can be measured
Types of Percent
• W/W percent or Weight/Weight percent
• W/V percent or Weight/Volume percent
• V/V percent or volume/volume percent
• When type of percent is not stated,
– Dry ingredients in a dry preparation are percent W/W
– Dry ingredients in a liquid are percent W/V
– A liquid in a liquid are percent V/V
Examples of pharmaceutical dosage forms in which the active
ingredient is often calculated and expressed on percentage basis
|
Percentage basis |
Examples of dosage forms |
|
Weight-in-volume |
Solutions (ophthalmic, nasal, otic, topical, large-volume |
|
Volume-in-volume |
Aromatic waters, topical solutions, emulsions |
|
Weight-in-weight |
Ointment, creams and gels |
Solving percentage
problems
IF no. of parts / 100 = THEN Amount of solute needed / Total
volume or weight of the product
Example:
How many grams of zinc oxide are needed to make 240 grams of
a 4% (w/w) zinc oxide ointment?
IF 4 g ZnO /
100 g Oint = THEN X g ZnO / 240 g
X = 4 X 240 / 100
= 9.6 g
Practice
problems
• How many milliliters of a 5% (w/v) boric acid solution can
be made from 20 grams of boric acid?
• How many milliliters of paraldehyde are needed to make 20
ml of a 10% (v/v) paraldehyde solution?
• How many grams of ephedrine sulfate are needed to make 120
ml of a 2% (w/v) ephedrine sulphate solution?
• How many grams of zinc oxide are needed to make 120 grams
of 20% zinc oxide paste?
• How many grams of potassium permanganate are required to
compound the following prescription:
Potassium permanganate 0.02%
Distilled water q.s. to 240 ml
• How many grams of holocaine hydrochloride and how many
grams of chlorobutanol are needed to compound this prescription?
Holocaine hydrochloride ½%
Chlorobutanol 1/3%
Distilled water q.s. to 60 ml
• If 5 g of a chemical is dissolved in enough water to make
the preparation measure one litre, what is the percentage strength of the
solution?
• How many milliliters of a 0.02% W/V solution can be made
from 2.5 g of a chemical?
• Normal saline solution contains 0.9% W/V NaCl. How many
grams of sodium chloride should be used to make 1.5 liters of normal saline?
• How many grams of glucose will be required to prepare 10
litres of 2.5% solution?
• An injection contains 50 ml of pentobarbital sodium in
each ml of the solution. What is the percentage of the final solution?
• How many grams of sucrose must be dissolved in 475 ml of
water to form a 65% W/V solution?
• An ear drop formula contains 54 mg of antipyrine and 14 mg
of benzocaine in each ml of the solution. Calculate the percentage strength of
each ingredient?
Summary
• Different percent preparations – %W/W, %W/V, %V/V
• Solving problems related to percentage calculations