Introduction
In chemistry, concentration is the amount solute in given amount of solution. There are varieties of measures available to define concentration of solution. The solute is the solid/ liquid / gas compound which being dissolved, whereas the solvent is the liquid / gas phase dissolving medium. The solution which consists of solute in large amount is considered as concentrated solution. And the solution prepared by very small amount of solute is called as dilute solution. The concept of concentration is crucial in organic chemistry because most of the organic reactions are performed in solution phase. Some of the sensitive reagents are used as solution for reaction purpose. For example n-Butyl lithium is used as molar solution in hexane or tetrahydrofuran (THF). Also hydrogen peroxide is used in the form of aqueous solution. Hence it is necessary to understand units of concentration in chemistry.
Here we will discuss about units of concentration which are frequently used.
1. Molarity
Molarity
is defined as number of moles of solute in one litre of solution. The molarity of
the solution is calculated by following formula;
Molarity = moles
solute/liter of solution
2. Normality
Normality
is a unit of concentration; it is defined as number of gram equivalents of
solute in one liter of solution. The normality of a solution is calculated by
the formula;
N
= equivalents of solute/liter of solution
But equivalent weight of
compound = Molecular Wt. / Valency
Therefore;
For example, if we take
100 g of sodium carbonate (Na2CO3) and make it to 1 liter
solution in water then what will be the normality of that solution?
We know that molecular
weight of sodium carbonate is 105.98 g/mol. It produces 2 Na+ and CO3-
ions in solution. The ionic charge on both ions is 2, which is valency. Therefore
normality is calculated by the formula;
3.
Weight percent
It is a ratio of mass of
solute and total mass of solution multiplied by 100. Weight percent of solute
is calculated by the formula;
For example; 10 g of
glucose is dissolved in 500 ml water. Then weight percent of glucose is
calculated by the formula;
4.
Volume percent
It is a ratio of volume
of solute and total volume of solution multiplied by 100.
For example; 15 ml of
ethanol is mixed with 50 ml of pure water. What is the volume percent of
ethanol in the solution?
5.
Mass/Volume percent
It is a ratio of mass
of solute and total volume of solution multiplied by 100.
For example; 75 ml solution of sucrose (C12H22O11) in water contains 5 g of sucrose. What is mass / volume percentage of sucrose in the solution?
6.
Parts per million (PPM)
It is unit of
concentration used for very dilute solutions. It is calculated as parts of the
solute per million parts of solvent. Suppose 0.005 g of sodium chloride (NaCl)
dissolved in 1000 ml of water. So concentration in ppm is calculated as;
7.
Mass per volume
It is a unit of concentration which is defined as ratio of mass of solute and liter of solution. It is expressed in gram per liter of the solvent. For example; 1 g sodium chloride (NaCl) dissolved in water to make final volume of 100 ml (0.1 L). So mass per volume component is calculated as;
8.
Molality
Molality
is a unit of concentration; it is defined as number of moles of solute in 1
kilo gram (Kg) of solvent. The main benefit of uses of this unit is that molality
only depends on the masses of solute and solvent. It does not affect by temperature
and pressure whereas other units such as molarity is affected by temperature
and pressure.
For
example; 100 g of sodium bicarbonate (NaHCO3) dissolved in 0.5 Kg of
water then molality of the solution is calculated as;
Moles of NaHCO3
= Mass / molecular wt = 100 g / 84.007 g/mol = 1.190 moles
Therefore;
9.
Mole fraction ( χ )
It is measure of
concentration of solution which is defined as ratio of moles of solute and
total moles of the solution. It is calculated by the formula;
For example, 10 g of
glucose (C6H12O6) dissolved in 100 g of pure
water, what will be the mole fraction of glucose in the solution?
Given:
Moles of glucose = 10 g
/ molar mass = 10 g / 180.16 g/mol = 0.055 mole
Moles of water = 100 g
/ molar mass = 100 g / 18.015 g/mol = 5.550 moles
Total moles of solution
= Mole of glucose + moles of water = 0.055 + 5.550 = 5.605 moles
That's all for this topic. If you have any questions please feel free to ask me in the comment box. Thank you..!
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