CONCENTRATION UNITS OF SOLUTIONS
There are various types of concentration units of solutions. They are as following:
(1) Percentage composition
(2) Molarity (M)
(3) Molality (m)
(4) Normality (N)
(5) Mole fraction (X)
(6) Parts per million (ppm)
(1) PERCENTAGE COMPOSITION
The parts of solute dissolved in 100 parts of solution is called percentage composition of solution.
The amounts of solute and solvent can be expressed in percentage composition by four different ways.
(a) Percentage weight / weight
(b) Percentage weight / volume
(c) Percentage volume / weight
(d) Percentage volume / volume.
(a) Percentage Weight / Weight:
It is the weight of a solute dissolved per 100 parts by weight of solution. or number of grams of solute dissolved per 100g of solution is called %W/W.
Example:
5% w/w sugar solution means 5 g of sugar is dissolved in 95g of solvent say water. Total mass of solution is 100 g.
Formula:
% by weight = 
× 100.
× 100.
(b) Percentage Volume / Weight:
The parts of solute by volume dissolved in 100 parts of solution by weight, is called percentage V/W or number of cm3 of the solute per 100g of the solution is called % V/W.
In such type of solutions, total volume of the solution, volume and mass of the solvent is exactly not known.
Example:
If 10 cm3 of alcohol is dissolved in water and the total weight of the solution is 100 g, then it is 10% V/W solution of alcohol in water.
(c) Percentage Volume / Volume:
Parts of solute by volume dissolved in 100 parts of solution by volume is called V/V percentage of that solution or number of cm3 of the solute per 100cm3of the solution is called % V/V.
In such solution, the total volume of the solution may not be necessarily equal to the sum of volumes of solute and the solvent.
Example:
When 12cm3 of alcohol is dissolved in water to make 100cm3 of solution then solution is called 12% by volume.
This unit of concentration is best applicable to the solution of liquid in liquid.
(d) Percentage Weight / Volume:
The parts of solute by weight dissolved in 100 parts of solution by volume is called W/V % of that solution or number of grams of the solute per 100cm3 of the solution is called W / V %.
Example:
10g of glucose is dissolved in 100cm3 of solution is 10% W/V solution of glucose.
(2) MOLARITY(M)
Number of moles of solute dissolved per dm3 of the solution is called molarity of solution. It is represented by M.
Examples:
(1) If we take 180g of glucose (C6H12O6) and add in it sufficient amount of water to make total volume of solution 1dm3 then it is called one molar solution of glucose.
(2) Similarly 342g of sucrose (C12H22O11) is dissolved in so much water to make total volume of solution one dm3. Then it is called one molar solution of sucrose.
In first case, volume of 180g of solute is small as compared to volume of 342g of sucrose. Also, in first case, large amount of water is required to make volume 1dm3 and in 2nd case small amount of water is required to make total volume 1dm3 of solution because volume of 342g of sucrose is large as compared to 180g of glucose.
Units:
Units of molarity are moles dm–3 or molar.
Formula:
Molarity = 
Molarity = 
× 
×
Molarity = 
Molarity = ×
×
Molarity = × 
×
(3) MOLE FRACTION (x)
The mole fraction of any component in a mixture is the ratio of the number of moles of that component to total number of moles of all the components present in mixture.
Example:
Let there be three components A,B,C making solution. The number of moles are nA, nB and nc respectively. Then mole fractions of components are represented by 
respectively and given by:
respectively and given by:
= 
and so on
Note:
(i) The sum of mole fractions of all the components is unity.
(ii) The mole fraction of one component is always less than unity.
(iii) The mole fraction when multiplied with 100 gives us mole percent.
(iv) The sum of mole percents of all the components in a mixture is 100.
(v) The mole percent of one component is always less than 100.
(vi) Mole fraction when multiplied by 100 then this is called mole percent.
(vii) This unit of concentration may be for any type of solution the gas in gas, liquid in liquid or solid in liquid etc. This unit is also applicable to a solution having more than two components
(4) PARTS PER MILLION (ppm)
It is defined as the number of parts (by weight or volume) of a solute present per million parts (by weight or volume) of the solution.
This unit is used for very low concentrations of solutions. e.g to express the impurities of substances in water.
Formula:
Parts per million (ppm) =
× 106
× 106
Parts per million (ppm) = 
× 106
× 106
INTERCONVERSION OF VARIOUS CONCENTRATION UNITS OF SOLUTION:
Sometimes, we get solution from chemical supply houses with concentration given in specific concentration unit. For example we are working with a solution whose molarity is given by the supplier, but we need to know its molality or w/w percentage. For such purpose, we need to convert one unit into other.
These conversions are usually done if we know:
(i) The formula masses
(ii) The densities of the solutes or solutions.
Following table shows the five important chemicals whose w/w%, molarities and densities are given. We convert these concentration units into each other.
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