Foundationsof
ililfiiy
EXAMPLES OF PHARMACEUTICAL CALCULATIONS
SIGNIFICANT FIGURES
1) ln each case, if the answer does not contain the proper number
of significant figures, rewrite it so
that allthe figures retained are significant:
a) 15.432 g x 0.26 = 4.01232 g
b) 0.2350 g. 0.5S = 0.42727 9
c) 1.25500 g + 0.650 g+0.1259 + 12.78900 g 14.81900 g
=
d) 16.23 mt x 0.75 = 12.1725 mt
e) 437.5 mt. 1.25 = 350.000 mt
2) what is the difference in meaning between a volume recorded as
423 ml and one recorded as
473.Q ml?
3) what is the difference in meaning between a weight recorded as
0.65 g and one recorded as
0.6500 g?
CALCULATION OF DOSES
1) lf the dose of a drug is 200 mg, how many doses are contained
in 10 g?
2) How many ml of a liquid would provide a patient with 3
medicine measures twice a day for g days?
3) If 0'05 g of a substance is used in preparing 125 tablets,
how many micrograms are represented in
each tablet?
4) If a preparation contains 5 g of a drug in 500 ml, how many grams
are contained in each 15 ml
dose?
5) How many grams of a drug substance are required to make
120 ml of a solution, each s ml
measure of which will contain 3 mg of drug substance?
6) The usual initial dose of chlorambucil is rso #g.q", of body weight once a day. How many
milligrams shourd be administered to a person weighing tg_
liq bz
7) lf a liquid medicine is to be taken three times daily, and if 180 ml
are to be taken in 4 days, how
many tablespoons should be prescribed of each doie?
Page 1 of 9
Foundations of
flffiilXc;
g) lf a cough syrup contains 0.24 g of codeine phosphate in 120 ml, how many milligrams are
contained in each teaspoonful dose?
g)
' A physician prescribes tetracycline suspension for a patient to be taken in doses of two
teaspoonfuls four times a day for 4 days and then one teaspoonful four times a day for 2 days. How
prescribed
many millilitres of the suspension should be dispensed to provide the quantity for the
dosage regimen?
a day.
' Lugol's solution contains 5 g of iodine per 100 ml and its usual dose is 0.3 ml three times
10)
How many milligrams of iodine are represented in the daily dose of the solution?
11) A physician prescribes 1.6 million units of penicillin daily for 7 days. lf 1 mg of penicillin G is equal
to 1600 units of penicillin, how many 250 mg tablets of penicillin G should be dispensed for the
prescribed dosage?
12) How many millilitres of an injection containing 20 mg of gentamicin in each 2 ml should be used in
filling a medication order calling tor 2,5 mg of gentamicin to be administered intramuscularly?
13) A physician ordered 1.5 milligrams of theophylline to be administered orally to a baby' How many
millilitres of theophylline elixircontaining 80 mg of theophylline per 15 ml should be used in filling
ihe medication order?
14) Rx Zinc oxide
Talc
Starch aa 5.0
Lanolin
Petrolatum aa ad 60.0
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Foundations of Pharmacy
Calculations
REDUCING AND ENLARGING FORMULAS
1) From the following formula, calculate the quantity of each ingredient required to make 240 ml of
Calamine Lotion:
Rx
Calamine 80 g
Zinc oxide 80 g
Glycerin 20 ml
Bentonite 250 ml
Lime water to make 2 000 ml
2) From the following formula, calculate the quantity of each ingredient required to make 1000 g of the
ointment.
Rx
Coaltar 5 parts
Zinc oxide 10 parts
Hydrophilic oint 50 parts
SPECIFIC GRAVITY AND DENSITY
1) lf 54.96 ml of an oil weighs 52.78 g, what is the specific gravity of the oil?
2) lf 250 ml of alcohol weighs 203 g, what is its density?
3) A 30 ml sample of sulphuric acid weighs 55 g. Calculate its density.
4) lf 150 ml of polyethylene glycolweighs 17A g, what is its specific gravity?
5) If a litre of sorbitol solution weighs 1285 g what is its specific gravity?
6) lf 500 ml of ferric chloride solution weighs 650 g, what is its specific gravity?
7) What is the weight of 100 ml of hydrochloric acid having a specific gravity of 1.16?
8) What is the weight of 300 ml of glycerin having a specific gravity of 1.25?
9) What is the volume in millilitres of 1 kg of sulphuric acid with a specific gravity of 1.83?
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Fou ndations of
[Xil;l??
1O) A formula for vanishing cream contains 750 g of glycerin. How many millilitres of glycerin (specific
gravity 1.25) should be used?
11) A cosmetic formula calls for 500 g of peach kernel oil (specific gravity 0.910) per 1000 g of the
finished product. How many millilitres of the oil should be used in preparing 5000 g of the formula?
PERCENTAGE CALCULATIONS
1) Calculate how many grams of NaGl are required to prepare 300 ml of a 10o/o (m/v) solution
2) lt 24 g boric acid is dissolved in sufficient alcohol to make a total volume of 120 ml, what is the
strength of the resulting solution (wiv)?
3) What is the % strength (m/v) of a solution of urea if 80 ml contains 12 g?
4) lf 12 ml peppermint oil was dissolved in sufficient alcohol to make 80 ml of solution, what would be
o/o?
the concentration of peppermint oil expressed as a
5) How many ml of a3o/o solution can be made from 27 g ephedrine sulphate?
6) lf 4 g peppermint oil is dissolved in 96 g alcohol, how would you describe the concentration (w/w) of
peppermint oil in this solution?
7) Rx Eucalyptus oil 2.5% vlv
Mineraloilad 30.0
8) How many mg of a red dye should be used in preparing 5 litres of a 0.01% solution?
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Foundations of Pharmacy
Calculations
9) How many grams of drug substance should be dissolved in 240 ml water to make a 4o/o (wlw)
solution?
10) How should you prepare 100 ml of a2% (w/w) solution of a drug substance in a solvent having a
specific gravity ol 1.25? Assume that the SG of the dilute solution is the same as the SG of the
solvent.
11) How many millilitres of liquefied phenol should be used in compounding the following prescription:
Rx Liquefied phenol 2.5%
Calamine Lotion ad 240.0 ml
12) How many g of drug substance are required to make 120 ml of a 20o/o (w/w) solution having a
specific gravity of 1.15?
RATIO STRENGTH
1) Express 0.02% as a ratio strength. 2) Express 1:4 000 as a'/o strength
3) How many milligrams of gentian violet should be used to make up 500 ml gentian violet solution with
a strength of 1:10 000?
4) Calculate how many milligrams of each must be added.
Rx Phenacaine hydrochloride 1 :100
Mercury bichloride 1:4000
Ophthalmic base ad 1og
5) A tetracycline syrup is preserved with 0,8% (w/v) of methyl paraben, 0.02% (w/v) of propylparaben
and 0.10% (Mv) of sodium metabisulfite. Express these concentrations as ratio strengths.
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Foundations of Pharmacy
Calculations
UNITS OF POTENCY
used to obtain 40 units of insutin?
(u-100 insulin
1) How many mi*,itres of u-100 insurin shourd be
contains 100 units Per ml)'
2)Howmanymillilitresofheparinsodiuminjectioncontainin.g.200,ooounitsinl0mlshouldbeusedto
obtain5,000heparinsodiumunitswhicharetoneaddedintoanintravenousdextrosesolution?
3)HowmanymillilitresofU40isophaneinsulinsuspensionshouldbeusedtoprovide2Sunitsof
insulin?
ergocalciferol represents 400 units of vitamin
D' how many 1'25 mg ergocalciferol
4)' lf 10 mcg of
units of vitamin D?
capsules wilt proviOe a dose of 200 000
micrograms represent one unit?
5) lf 1 mg of penicillin V represents 1530 units of penicillin' how many
DILUTION AND CONCENTRATION
what will the
saricyrate in arcohor is diruted to 1500 ml,
1) rf s00 mr of a 15% (v/v) sorution of methyr
(v/v) be?
Percentage strength
(m/m) ammonia
water can be made from 1800 g of 28%
2) How many grams of 10% (m/m) ammonia
water?
3)Howmanymillilitresofal:400(m/v)stocksolutionshouldbeusedtomake4litresofal:2000(miv)
solution?
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Foundatioirs of Pharmacy
Calculations
4) How many millilitres of a 1T% (m/v) concentrate of benzalkonium chloride should be used in
preparing 300 ml of a stock solution such that 15 ml diluted to a litre will yield a 1:5000 solution.
5) How many millilitres of water should be added to 300 ml of a 1:750 (m/v) solution of bbnzalkonium
chlqide to make a 1:2500 (m/v) solution.
6) How many millilitres of 96% (m/m) sulphuric acid having a specific gravity af 1.84 are required to
make 1000 ml of diluted sulphuric acid, 10% (m/v)?
ALLIGATION
1) A 400 g powder containing gyo A is to be made from a powder containing 10o/o A and a second
powder containing 2% A.
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Foundations of Pharmacy
Calculations
6) How many mls of 0.5% soldium sulphate solution should be mixed with 5% sodium sulphate
solution to make 1 litre of a 2% solution?
7) How many mls of 0.45% (m/v) sodium hypochlorite solution can be prepared from 800 ml of a
1 1.25o/o (m/v) solution?
8) How many mls of sterile distilled water must be added to 10 L of a 50% (m/v) sterile dextrose
solution to reduce the concentration to 30% (m/v)?
9) Howmanymlsof watershouldbeaddedtol00ml of a1:125(m/v)solutiontomakeasolution
such that 25 ml diluted to 100 ml will yield a 1:4000 dilution?
10) A pharmacist mixes together 236 ml af a 5% solution of sodium chloride and 1764 ml of a 1:800
solution of sodium chloride.
(a) Calculate the percentage strength (m/v) of the resulting mixture
(b) How many mls of the above solution should be added to 250 ml of a 1:1000 solution such that
50 ml diluted to 200 ml will yield a 1:2000 solution?
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Foundations of Pharmacy
Calculations
2) ln what proportions should alcohols of 95% and 50% strengths be mixed to make 7070 alcohol?
3) You have 200 mlof 35% methanolsolution
l) How much water must be added to obtain a25o/o solution?
ii) How much pure methanol must be added to obtain a 45o/o solution?
4) How many grams of 2.5Yo hydrocortisone cream should be mixed with 0.25% cream to make 540 g
of a 1% hydrocortisone cream?
5) What is the strength of NaCl solution obtained by evaporating 800 g of a 10% (m/m) solution to
250 g?
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