Methods for expressing concentration
Content
• Methods for expressing concentrations in volumetric
analysis
• Normality and molarity
• Calculations
• Types of volumetric analysis
Learning
Objectives
At the end of this
lecture, the student will be able to
• List the methods for expressing concentrations in
volumetric analysis
• Different methods for expressing concentrations in
volumetric analysis
• Define Normality and molarity
• Discuss about the preparation of different concentrations
of solution in terms of normality and molarity
Concentrations
Introduction:
A solution is a
mixture of a solute and a solvent
A solvent: is a
substance that dissolves other materials
A solute: is the substance that dissolves in the
solvent
A concentrated
solution contains a large amount of solute per litre of solution e.g.
strong coffee
A dilute solution
contains a small amount of solute per litre of solution e.g. weak coffee
The concentration of a solution is the amount of solute that
is dissolved in a given volume of solution
There are several
ways of expressing the concentration of a solution:
1) Percentage of
solute – 3 forms
2) Parts per million
(ppm)
3) Moles of solute
per litre of solution (MOLARITY)
4) Gram equivalent
of solute per litre of solution (NORMALITY)
5) Moles of solute
per kg of solution (MOLALITY)
1. Percentage of Solute
This method of expressing concentration is usually used in
many household solutions and in medicine
There are 3 ways in which the percentage of solute in a
solution is expressed:
a) Percentage weight per weight (w/w)
b) Percentage weight per volume (w/v)
c) Percentage volume per volume (v/v)
a) Percentage weight per weight
(w/w):
This is the number of grams of solute per 100g of solution
e.g.
10% w/w NaCl → 10g of sodium chloride per 100g of solution
2% w/w salicylic acid ointment → 2g of salicylic acid per
100g of ointment
b) Percentage weight per volume
(w/v):
This is the number of grams of solute per 100cm3 of solution
e.g.
10% w/v NaCl → 10g of sodium chloride per 100cm3 of solution
5% w/v NaCl → 5g of NaCl in 100cm3 of solution
*Note: usually the units are grams per litre (g/L) therefore
you would have to bring it to a litre*
c) Percentage volume per volume
(v/v):
This is the number of cm3 of solute per 100cm3 of solution
e.g.
5% v/v ethanoic acid solution → 5cm3 of ethanoic acid per
100cm3 of ethanoic acid
13% v/v ethanol
solution → 13cm3 of ethanol (alcohol) per 100cm3 of ethanol
2. Parts
Per Million (ppm)
This method of expressing the concentration of a solution is
only used for very dilute solutions i.e. when dealing with very low
concentrations of substances.
• This is the number of milligrams per litre (mgL-1)
*Note: 1 Litre of
water has a mass of 1 million milligrams*
• So, can say 1mg/L = 1 mg per million mg
= 1 ppm
• Example: the concentration of chlorine in water is 2 ppm
this means there are 2 mg of chlorine per litre of water
3) Moles of
Solute per Litre of Solution (MOLARITY)
Remember:
One mole of a substance is the amount of that substance
which contains 6 x 1023 particles (atoms, ions, molecules) of that substance
Mass of 1 mole of an element = Relative Atomic Mass in grams
e.g.
1 mole of Na = 23 g
1 mole of Mg = 24g
1 mole of hydrogen=1g
The most important way of expressing the concentration of a solution
is in terms of moles per litre of solution (molarity)
Definitions:
• The Molarity of a solution is the number of moles of
solute per litre of solution
• A 1 molar solution is a solution which contains one mole
of solute per litre of solution (1 M)
also,
- A solution which contains 5 moles of solute in a litre of
solution is said to be 5 molar (5M)
- A solution which contains 0.5 moles of solute in a litre
of solution is said to be 0.5 molar (0.5M)
• Symbols Used:
- M
- mol/L or mol L-1
Remember:
Mass of Substance per litre
No. of Moles of Substance (Molarity) = ------------------------------------------
Molar
Mass/Molecular weight
Concentration
Examples:
-1 mol/L NaOH = 40g NaOH (Mr NaOH = 40) per litre of
solution
- 2mol/L NaOH = 80g NaOH per litre of solution
- 0.5mol/L NaOH = 20g NaOH per litre of solution
- 0.1 M (decimolar) NaOH = 4g NaOH per litre of solution
Complete the
following:
1M Sulphuric acid = ________________________
0.5M Potassium hydroxide = ________________________
3M Nitric acid = ________________________
5M Sodium thiosulphate = ________________________
4) Gram
Equivalent of Solute per Litre of Solution (NORMALITY)
The most important way of expressing the concentration of a
solution is in terms of gram equivalent per litre of solution (Normality)
Definitions:
• The Normality of a solution is the number of gram
equivalent weight of solute per litre of solution
•A 1 Normal solution is a solution which contains one gram
equivalent weight of solute per litre of solution (1 N)
also,
- A solution which contains 5 gram equivalent weight of solute
in a litre of solution is said to be 5 Normal (5N)
- A solution which
contains 0.5 gram equivalent weight of solute in a litre of solution is said to
be 0.5 normal (0.5N)
• Symbols Used:
- N
- Gram equivalent/L
Remember:
Gram equivalent weight of Substance per litre
No. of gram equivalent weight of a
Substance (Normality) = ------------------------------------------------
Equivalent weight
Concentration
Examples:
-1 N/L NaOH = 40g NaOH (Mr NaOH = 40) per litre of solution
- 2 N/L NaOH = 80g NaOH per litre of solution
- 0.5 N/L NaOH = 20g NaOH per litre of solution
- 0.1N (decimolar) NaOH = 4g NaOH per litre of solution
Complete the
following:
1N Sulphuric acid =
0.5N Potassium hydroxide =
3N Nitric acid =
5 N Sodium thiosulphate=
5) Moles of
solute per kg of solution (MOLALITY)
Molality is defined as the “total moles of a solute
contained in a kilogram of a solvent.”
Molality is also known as molal concentration.
It is a measure of solute concentration in a solution.
The standard unit of
molality is moles per kilogram (mol kg−1). It also an SI unit. mol kg−1 is also
called molal. In other words, 1 molal = 1 mol kg−1.
• Symbols Used:
m = molality
mol = moles of solute
kg = kilogram of solvent
Moles
of solute
m = --------------------------
Kilogram
of solvent
Difference
between Molarity and Molality
|
Molarity |
Molality |
|
The molarity of a
given solution is defined as the total number of moles of solute per litre of
solution. |
Molality is defined as
the total moles of a solute contained in a kilogram of a solvent. |
|
Mathematical expression is M =
number of moles of the solute /Volume of solution given in terms of litres. |
The mathematical
expression is m = Numbers of moles of
solute/Mass of solvent in kgs m = (g ✕ 1000)/(W ✕ m). |
|
Depends on the volume
of the whole solution. |
Depends on the mass of
the solvent. |
|
Unit sign expressed as
(M). |
Unit sign expressed as
(m). |
|
Molarity has a unit of
moles/litre. |
Molality has units of
moles/kg. |
Summary:
• Concentrations are expressed in terms of different
methods: Percentage of solute – 3 forms, Parts per million (ppm), Normality,
Molarity
• Molarity: No of gram moles of solute per litre of solvent
• Normality: No of gram equivalent of solute per litre of
solvent
• Molality : Moles of solute per kg of solution
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