LESSON 2
PHARMACEUTICAL CALCULATIONS II
Section I. SPECIFIC GRAVITY
2-1. INTRODUCTION
Specific gravity often becomes a part of the solution to a pharmaceutical
calculation. Hence, the main use of specific gravity is to solve for a liquid's volume
when the weight of the liquid is known. Because of the difficulty which may be
encountered in trying to weigh a liquid, it is often advantageous to calculate the liquid's
volume and measure it in a graduate as opposed to weighing it.
2-2. DEFINITION
a. Specific Gravity. Specific gravity is the ratio of the weight of a substance to
the weight of an equal volume of distilled water at 25C.
b. At 25C and 1 atmosphere of pressure, one milliliter of distilled water weighs
one gram. Therefore, the specific gravity of water is established as one.
c.
Formula.
Specific gravity =
Weight of the substance
Weight of an equal volume of water
d.
Because one milliliter of water weighs one gram:
Specific gravity =
Number of grams of the substance
Number of milliliters of the substance
e. Specific gravity has no units. Because specific gravity has no units, only the
numbers must be placed in the formula providing the units of weight and volume are
grams and milliliters. If units are other than grams and milliliters, using the conversion
factors as discussed in the metric system and common systems of measure should
change them.
f. Example Problem. What is the specific gravity of 10 milliliters of mineral oil,
which weighs 8.5 grams?
Specific gravity = # of grams of substance
# of milliliters of the substance
Specific gravity= 8.5
Sp gr= 0.85
10
MD0802
2-2
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