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```Advantages and disadvantages of solutions
• Easy to swallow, especially for pediatric and geriatric patients
• Dissolved drug is immediately available for GI tract, thus onset
of therapeutic response is fast.
• Uniform distribution of the drug in the dosage form
• Flexible dosing according to the weight and age of the patient
• Immediate dilution in the GI tract fluids avoids irritation
• Bulky, inconvenient for transport and storage and glass is
breakable
• Stability is poor due to hydrolysis
• Active drug and excepients may chemically interact more readily
in solution than in solid dosage form
• Suitable medium for microbial growth
• Unpleasant taste will be amplified when drug is in solution
A. Solid in liquid solubility e.g., NaCl or Sugar in water
Concentration Expressions of Solutions
Molarity(M): Moles of solute in 1000 ml of solution
Molality* (m): Moles of solute in 1000 gm of solvent
Mole Fraction(A): Ratio of the moles of one
constituent (solute or solvent) of a solution to the
total moles of all constituents (solute and solvent).
Mole percent: is obtained by multiplying mole
fraction by 100.
Number of Moles: Weight in grams divided by
molecular weight in gm / mole
* Note that molality is the only concentration unit in which
denominator contains only solvent information rather than
solution.
Other concentration expression: % Concentration
mass solute
• % (w/w) =
mass solution
x 100
mass solute
• % (w/v) =
volume solution
x 100
volume solute
• % (v/v) =
volume solution
x 100
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Example:
An aqueous solution of ferrous sulphate (FeSO4) was prepared
by adding 41.5 gm of FeSO4 to enough water to make 1 L of
solution at 18o C. The density of the solution is 1.0375 gm/mL,
and the molecular weight of FeSO4 is 151.9 gm/mol and
molecular weight of H2O is 18.02 gm/mol .
Calculate
A) the Molarity FeSO4;
Number of moles of FeSO4 = Weight ÷ molecular weight
Number of moles of FeSO4 = 41.5gm ÷ 151.9 gm/mol = 0.2732
moles
Molarity (M) = No. of moles in liters of solution
Molarity = 0.2732 ÷ 1 L = 0.2732 M
B) the Molality FeSO4;
Molality = No. of moles in 1000 gm of solvent
No. of moles = 0.2732 moles
Volume of solution= 1000 ml, But weight of solvent is not
known.
Weight = volume X density
Weight of solution = 1000 ml X 1.0375 gm/ml = 1037.5 gm
Weight of solvent = 1037.5 gm- 41.5 gm = 996 gm =0.996 kg
Molality = 0.2732 ÷ 0.996 kg= 0.2743 m
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Example-continued:
An aqueous solution of ferrous sulphate (FeSO4) was prepared by adding 41.5 gm
of FeSO4 to enough water to make 1 L of solution at 18o C. The density of the
solution is 1.0375 gm/mL, and the molecular weight of FeSO4 is 151.9 gm/mol and
molecular weight of H2O is 18.02 gm/mol .
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C) the mole fraction and mole percent of water.
Mole fraction (X1) of water = moles of water/ (moles of water
+moles of FeSO4)
Moles of water = 996 gm ÷ 18.02 gm/mol = 55.27 moles
Mole fraction (X1) of water = 55.27 / (55.27 + 0.2732 )= 0.9951
Mole percent of water = mole fraction X 100
Mole percent of water = 0.9951 X 100 = 99.51%
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D) the mole fraction and mole percent of FeSO4;
Mole fraction (X2) of FeSO4 = moles of FeSO4 / (moles of water
+moles of FeSO4)
Mole fraction (X2) of FeSO4 = 0.2732 / (55.27 + 0.2732 ) = 0.0049
mole percent of FeSO4 = mole fraction X 100
mole percent of FeSO4 = 0.0049 X 100 = 0.49%
N.B
Sum of X1 and X2 = 0.9951+0.0049 = 1.00
• Example-continued:
• An aqueous solution of ferrous sulphate (FeSO4) was prepared by
adding 41.5 gm of FeSO4 to enough water to make 1 L of solution at
18o C. The density of the solution is 1.0375 gm/mL, and the molecular
weight of FeSO4 is 151.9 gm/mol and molecular weight of H2O is 18.02
gm/mol .
• E) the percent w/w of FeSO4.
• the percent w/w of FeSO4 = (weight of FeSO4 ÷ weight of solution)X100
• the percent w/w of FeSO4 = (41.5 gm ÷ 1037.5)X100 = 4% w/w
• F) the percent w/v of FeSO4.
the percent w/v of FeSO4 = (weight of FeSO4 ÷ volume of solution)X100
the percent w/v of FeSO4 = (41.5 gm ÷ 1000 ml)X100 = 4.15% w/v
• Other descriptive terms of solubility:
Very soluble
one gm of solute / one ml of solvent
Freely soluble one gm of solute/ from 1 to 10 mls of solvent
soluble
one gm of solute/ from <10 to 30 mls of solvent
sparingly soluble one gm of solute/ form < 30 to 100 mls of solvent
slightly soluble
one gm of solute/ form < 100 to 1000 mls of solvent
very slightly soluble one gm of solute/ form < 1000 to 10000 mls of solvent
practicaly insoluble or insoluble one gm of solute/ more than 10,000 mls of
solvent
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