LAB 4
Density, Specific gravity, Specific volume, and
Expressions of concentration
Density
• Density is mass per unit volume of a substance.
• Example if 10 ml sulfuric acid weighs 18g, its density is?
Density = 18(g)/ 10(mL) = 1.8 g/mL
Specific Gravity
• Specific gravity: is a ratio, weight of substance /
weight of equal volume of standard substance
have the same temperature.
• For specific gravity of liquid and solids, water
used as standard.
• 10 mL of H2O = 10 g (weight)
Specific Gravity
• Substance that have specific gravity less than 1 are
lighter than water
• Substance that have specific gravity greater than 1 are
heavier than water
• Example if 54.96 mL of an oil weighs 52.78 g, what is the
specific gravity of the oil ?
Specific gravity of oil = 52.78 g/ 54.96 g = 0.9603
Specific Gravity
• The following equation may be used to convert the
volume of substance or pharmaceutical preparation to its
weight:
g = mL x sp gr
• The following equation may be used to convert the weight
of substance or pharmaceutical preparation to its volume:
mL = g
sp gr
Specific Volume
• Example: calculate the specific volume of a syrup, 91 mL
of which weighs 107.16 g
107.16 g of water measures 107.16 mL
specific volume = 91/107.16 = 0.849
Percentage
• Percent % mean in a hundred
• Percentage means rate per hundred
Example 12.5% = 12.5 /100, or 0.125
Percentage
• Percent weight-in-volume (w/v): the number of grams of a
constituent in 100 mL of solution or liquid preparation
• Percent weight-in-weight (w/w): the number of grams of a
constituent in 100 g of solution or preparation.
• Percent volume-in-volume (v/v): the number of milliliters
of a constituent in 100 mL of solution or liquid preparation
Percentage
• Example how many grams of dextrose are required to
prepare 4000 mL of a 5% solution?
5g
100mL
x
4000mL
x = 5 x 4000 / 100 = 200g
Percentage
• Example: 5 g of glycerin in 100 mL syrup, express it as %
v/v. (sp gr 1.25)
5 g/ 1.25 = 4 ml
4 % v/v
Ratio Strength
• Ratio strength is another way of expressing percentage
strenght.
Example: 1% w/v solution is equivalent to 1:100 w/v ratio
strength
• The concentrations of very weak pharmaceutical
preparations (usually weight-in-volume) often are
expressed in terms of their ratio strengths.
Example: 1: 10,000 w/v means 1 gram of solute in 10,000
mL of solution
Practice problems
1. In preparing 250 mL of a certain lotion, a pharmacist
used 4 mL of liquefied phenol. What was the
percentage (v/v) of liquefied phenol in the lotion?
2. How many grains of atropine sulfate should be used in
compounding the following prescription?
Rx
Atropine sulfate
2%
Distilled water q.s. Ad
Ʒiv
Sig. use in the eye
3. If 250 mL of alcohol weighs 203 g. what is the density?
4. If a liter of cough syrup weighs 1285 g, what is its
specific gravity?
Practice problems
5. The specific gravity of alcohol is 0.815. what is its
specific volume?
6. If 100 mL of pharmaceutical preparation contains 20 µL
of 50% w/v solution of Benzalkonium chloride, what is
the percentage strength of that agent in the solution?
7. How many grams of a drug substance should be
dissolved in 240ml of water to make 4% w/w solution?
8. Five pints of diluted hydrochloric acid weighs 2.79 kg.
calculate its specific gravity
Practice problems
9. How many grams of potassium permanganate should
be used in preparing 500 mL of 1:2500 solution?
10. How many milligram of hydrocortisone should be used
in compounding the following prescription?
Rx
Hydrocortisone
1/8%
hydrophilic ointment ad 10g
Sig. apply BID
11. A pharmacist incorporate 6 g of coal tar into 120 g of
5% coal tar ointment. Calculate the percentage w/w of
coal tar in the finished product
12. Two 10 mL samples of urine have specific gravities
1.003 and 1.03. what is the difference in weight, in
milligrams, between the two samples?
Practice problems
13. What is the ratio strength w/v of a solution made by
dissolving 2 tablets, each containing 0.125 g of mercury
bichloride, in enough water to make 500 mL
14. How many mL of a 1:400 w/v stock solution should be
used to make 4 liters of 1:2000 w/v solution?
15. How many grams of drug substance are required to
make 120mL of 20% w/w solution having a specific
gravity of 1.15
Home work
1. A piece of copper metal weighs 53.6 g and has a
volume of 6 minims. Calculate its dinsety.
2. If 1000 mL of syrup with specific gravity of 1.313 contain
850 g of sucrose, what its percentage strength w/w?
3. If chloroform has a specific gravity of 1.476, what is its
specific volume?