Pharmacy calculations notes are essential study materials for pharmacists, ensuring they have the knowledge and skills to safely and accurately dispense medications to patients. These notes provide a structured approach to solving complex dosage and medication preparation problems encountered in da...
BPharm 1 2021
Pharmaceutical Calculations
MPIT011
Ratio and Dilution/concentration
You need to revise the following to be able to do these calculations:
Increasing and decreasing of formulae
The meaning of solute, solvent and solution
Calculation of the number of moles
mass m
eg no of mol = or n =
relative molecular mass RMM
The meaning of density where density is mass/unit volume or
the mass in grams of 1 millilitre of the substance
m
ie ρ =
v
Concentrations eg a) mol/1000ml or mol/dm 3 b) % m/v c) % v/v d) % m/m
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Concentration is sometimes expressed as parts of a component in a product
Eg NaCl (s)in a solution
NaCl = 1 in 50 means 1 part of NaCl (s) in 50 parts of the product
ie 1g of NaCl in 50ml of SOLUTION NaCl (aq)
or we can say a solution is diluted 1 part in 50
or 1 to 50
A solid associated with a solid
or the mass of the component in the mass of the product
eg 1 part in 10 of aspirin means 1g of aspirin in 10g of powder mass
A liquid associated with a liquid
or the volume of a liquid component in the volume of product
eg 1 part of ethanol in 20 parts of SOLUTION
or 1ml ethanol in 20 ml of solution
A gas associated with a liquid
eg 1 part CO 2 in 1000 parts H 2 O
Or 1g of CO 2 in a 1000g of H 2 O
Remember also concentration expressed as a % = the proportion of a specific item x 100
amount of item x 100 amount of solute x 100
eg % w/w = , %w/v = and
total amount amount of solution
volume of component x 100
%v/v =
volume of solution
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, Examples
1)How many grams of coal tar are there in 300g of coal tar and zinc ointment containing
10%w/w coal tar?
Using the formula for %
quantity of coal tar x 100
%w/w =
total quantity of ointment
quantity of coal tar = %w/w x total quantity of ointment
10 x 300g
=
100
= 30g coal tar
Using ratios
10%w/w coal tar ointment means
10g coal tar in 100g of ointment
x g coal tar in 300g of ointment
300 x 10
x = 30g coal tar
100
2a) How many milliliters of ethanol are there in a 400ml ethanol solution if the dilution is 1
in 32 of ethanol solution?
b) Express 1 in 32 ml of ethanol as a %v/v concentration
c) Describe how you would make up the 400ml ethanol solution in (a) practically
1 in 32 of ethanol means
1 ml of ethanol in 32 ml of ethanol solution
x ml of ethanol in 400 ml of solution
400ml x 1 ml
x = = 12,5 ml of ethanol
32ml
b) a 1 in 32 dilution means
1ml of ethanol in 32 ml of solution
x ml of ethanol in 100ml of solution
100 x 1
x = %v/v = 3,125 %v/v
32
c) To make this up practically, take 12,5ml of absolute ethanol (100%v/v ethanol) and add to
350ml of purified water. Allow to cool because the reaction is exothermic and the volume will
contract on cooling. When this has cooled make up to the 400ml mark in a measuring cylinder.
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