(3) Example #3: How many milliliters of paraldehyde are needed to make
120 ml of a 10% (v/v) paraldehyde solution?
_______ = ________
Answer: 12 ml of paraldehyde
NOTE: Because all of the units involved in this problem are the same (ml), they must
be labeled as to what they represent. A 10% paraldehyde solution
contains 10 milliliters of paraldehyde in each 100 ml of solution and the first
ratio of the proportion should indicate this (see below). The question asks,
"How many milliliters of paraldehyde are needed?" therefore, the X value must
be placed in the proportion opposite the 10 ml of paraldehyde in the first ratio.
The 120 ml represents final solution.
10 ml paraldehyde
X ml paraldehyde
100 ml of solution
120 ml of solution
100 X =
12 ml of paraldehyde
c. One percent Method: The 1 percent Method is used only to find the amount
of active ingredient when the final volume or weight of the preparation is known. This
method cannot be used to calculate the amount of preparation that can be made when
the percentage strength and the amount of active ingredient is known.
1 percent Method:
(1 percent of the total amount of preparation) X (number of percent) =
The amount of active ingredient)
NOTE: 1 percent of the total amount of preparation can be found by moving the
decimal point on the total amount of preparation two places to the left.
(1) Example #1: How many grams of ephedrine sulfate are needed to make
120 ml of a 2% (w/v) ephedrine sulfate solution?
Find the total amount of preparation:
The total amount is 120 ml