# solutions and methods for expressing concentration

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```					  SOLUTIONS AND DIFFERENT METHODS OF EXPRESSING
CONC. OF SOLUTION
Contents:
1.   Solution
2.   True solution
3.   How solution are characterized
4.   Types of solution
5.   Conc. And diluted solution
6.   Different methods of expressing conc. Of solution
(A). Mass % or % by mass
(b) % weight by volume
(c) % mass by volume
(d) strength of conc
(e) parts per million (ppm)

Solution: A solution is a homogeneous mixture composed of two or more substances. In such a
mixture, a solute is dissolved in another substance, known as a solvent. Solutions are groups of
molecules that are mixed up in a completely even distribution. Solutions have an even
concentration throughout the system.
A solution is a mixture of materials, one of which is usually a fluid. A fluid is a material that
flows, such as a liquid or a gas. The fluid of a solution is usually the solvent. The material other
than the solvent is the solute.

Examples: A solution of water and sugar is called syrup .
A solution of sodium chloride (common table salt) in water is called brine.
A solution of carbon dioxide in water is called seltzer,
A solution of ammonia gas in water is called ammonia water.

True Solution: A true solution is a homogeneous solution in which the solute particles have
diameters less than 10-7cm. i.e., the solute particles are of molecular dimensions. The particles
are invisible even under powerful microscopes.
A true solution is where you can't differentiate between the solute and solvent molecules even at
the microscopic level. It is completely homogeneous.

For example, sodium chloride in water is a true solution. Most ionic compounds form true
solutions in water. Organic compounds like sugar and urea also form true solutions in water.
How solutions are characterized:
The solute particles in a solution are generally of molecular size or smaller, much smaller than
those in a colloid or a suspension. The solute particles cannot be observed even with an ultra
microscope. They do not settle out from the solvent on standing, and they cannot be separated
from the solvent by physical means, such as filtration or centrifugation. On the other hand, a
solution differs from a compound in that its components can occur in continuously varying
proportions, within certain limits (although within a given solution they are present in the same
proportions throughout the solution), while the components of a compound can occur only in
certain fixed proportions.

The addition of solute affects the boiling point, freezing point, and vapor pressure of the
solution, in general raising the boiling point, depressing the freezing point, and lowering the
vapor pressure (Raoult’s law). A number of substances (acids, bases, and salts) exhibit
characteristic behavior in aqueous solution. These substances dissociate in water to form
positive and negative ions that enable the solution to conduct electricity. Such solutions are
called electrolytic.

The proportion of solute to solvent in a given solution is expressed by the concentration of the
solution. Concentrations may be stated in a number of ways, such as giving the amount of solute
contained in a given volume of solution or the amount dissolved in a given mass of solvent. A
solution having a relatively high concentration is said to be concentrated, and a solution having a
low concentration is said to be dilute.

In many solutions the concentration has a maximum limit that depends on various factors, such
as temperature, pressure, and the nature of the solvent. The maximum concentration is called the
solubility of the solute under those conditions. When a solution contains the maximum amount of
solute, it is said to be saturated; if it contains less than that amount, it is unsaturated.

The most obvious factor affecting solubility is the nature of the solvent. Ordinary table salt
(sodium chloride) is soluble in water, but only slightly soluble in ethanol, and insoluble in
diethyl ether. Temperature is also important in determining solubility. Solids are usually more
soluble at higher temperatures; more salt will dissolve in warm water than in an equal amount of
cold water. Graphs showing the solubility of different solids as a function of temperature are
called solubility curves and are very useful in chemical analysis. Solubility also depends on
pressure, especially in the case of gases, which are more soluble at higher pressures.

Under certain conditions a solution may be made to contain more solute than a saturated solution
at the same temperature and pressure; such a solution is called supersaturated. If even a single
crystal of undissolved solute is added to a supersaturated solution, all the excess solute above the
normal solubility concentration will immediately crystallize out of the solution.
Types of solution:
At the molecular level, molecules and ions of a solute are completely mixed with and interact
with those of the solvent when a solute dissolves in a solvent. This type of mixing is
homogeneous because no boundary is visible in the entire solution. In a mixture, differences may
exist between regions or parts of the whole system.

Material exists in three states: solid, liquid, and gas. Solutions exist in all these states:

1. Gaseous mixtures are usually homogeneous and all gases mixtures are gas-gas solutions.
For quantitative treatment of this type, of solutions, we will devote a unit to gases. The air is a
natural gas solution, but its water and carbon dioxide contents may vary depends on the
temperature and places.

2. When molecules of gas, solid or liquid are dispersed and mixed with those of liquid, the
homogeneous (uniform) states are called liquid solutions. Solid, liquid and gas dissolve in liquid
to form liquid solutions. In general, the terms solution and liquid solution are synonymous.
Gases and liquid solutions have attracted the attention of most chemists, while material scientists
and engineers are more interested in the manufacture and properties of solid solutions.

3. Many alloys, ceramics, and polymer blends are solid solutions. Within certain range, copper
and zinc dissolve in each other and harden to give solid solutions called brass. Silver, gold, and
copper form many different alloys with unique colors and appearances. Alloys and solid
solutions are important in the world of materials. The research, development, manufacture, and
production of these material are big business, and a company for example, Standard Alloys, may
concentrate on some aspects of these materials.
Table for Types of Solutions:

S.No.                Solute                Solvent               Types of             Examples
Solution

Solid Solutions

Solid                   Solid               Solid in solid          All alloys like
brass, bronze, an
1.                                                                                   alloy of copper and
gold, etc.
Liquid                   Solid              Liquid in solid           Amalgam of
mercury with Na,
2.                                                                                      CuSO4, 5H2O,
FeSo4, 7H2O
Gas                   Solid               Gas in solid        Solution of H2 in
Pd, dissolved gases
3.                                                                                    in minerals.

Liquid Solutions

4.                   Solid                Liquid              Solid in Liquid      Sugar solution,
salt solution, I2 in
CCl4
5.                   Liquid               Liquid              Liquid in Liquid     Benzene in
toluene, alcohol
in water,
6.                   Gas                  Liquid              Gas in Liquid        CO2 in water,
NH3, in water
etc.

Gaseous Solutions

7.                   Solid                Gas                 Solid in gas         Iodine vapors in
air, camphor
vapors in N2.
8.                   Liquid               Gas                 Liquid in gas        Water vapors in
air, CHCl3 vapors
in N2
9.                   Gas                  Gas                 Gas in gas           Air (O2 + N2)

Concentrated and Diluted solutions:
Concentrated solutions: A concentrated solution is one in which a lot of the water is removed or
there is as much of the chemical dissolved in the water as the water can hold. If you think of
pouring sugar into tea or coffee, it is the point where the sugar can still dissolve. Once you can see
the grains of sugar, the tea would be over saturated. Concentrated solution is a solution that
contains a large amount of solute relative to the amount that could dissolve.

Dilute solutions: A dilute solution is a solution that isn't so strong anymore because it has been
mixed with water or some other substance. Some medicines are diluted by the pharmacists before
they are released to patients. Solution in which the sum of amount fractions of all the solutes is
small compared to 1.

Different methods of expressing concentration of solution:
1. Mass percentage or % by mass: Mass (or weight) percentage (% w/w) is one of the most
often used ways of expressing concentrations. It is defined as

Mass of solution (in case of simple solution containing only one solvent and one solute) is

so we can formulate the definition in slightly different way:

Weight percentage is the only percentage concentration that is always unambiguous. Note that it
is expressed in % units (as opposed to % w/v).

2. Percentage weight by volume: Another variation on percentage concentration is
weight/volume percent or mass/volume percent. This variation measures the amount of solute
in grams but measures the amount of solution in milliliters. An example would be a 5%(w/v)
NaCl solution. It contains 5 g of NaCl for every 100. mL of solution.

Volume percent =       weight of solute (in g) X 100
volume of solution (in mL)

Because of the different units in the numerator and denominator, this type of concentration is not
a true percentage. It is used as a quick and easy concentration unit because volumes are easier to
measure than weights and because the density of dilute solutions is generally close to 1 g/mL.
Thus, the volume of a solution in mL is very nearly numerically equal to the mass of the solution
in grams.

3. Percentage mass by volume : Mass-volume (or weight-volume) percentage (% w/v) is
defined as

So for example solution containing 5g of substance in 100mL is 5% w/v.

4. Parts per million (ppm parts per milion (106)): "Parts per" is a convenient notation used for
low and very low concentrations. Generally speaking it is very similar to weight by weight
percentage - 1% w/w means 1 gram of substance per every 100 g of sample.

Parts per million ( ppm ) is commonly used as a measure of small levels of pollutants in air,
water, body fluids, etc. Parts per million is the mass ratio between the pollutant component and
the solution and ppm is defined as
ppm = 1,000,000 mc / ms     ,     where

mc = mass of component (kg, lbm)

ms = mass of solution (kg, lbm)

In the metric system ppm can be expressed in terms of milligram versus kg where

•   1 mg/kg = 1 part per million

Alternatively mass related units to measure very small concentration levels used are

•   ppb - parts per billion (1 / 1,000,000,000)
•   ppt - parts per trillion (1 / 1,000,000,000,000)

An alternatively mass related unit to measure larger concentration levels are weight percent
which can be expressed like

weight percent = 100 mc / ms

5. Strength of concentration: Strength is defined as the degree of ionization or dissociation of
the acid or base in aqueous solution. It can be measured using the equilibrium constant
expression for the dissolving of the substance in water. The degree of ionization is the extent
to which the acid, or ionic solids become ions. The equilibrium constant expression (Keq) for
acid are termed Ka and for bases termed Kb.

Comparing strong to weak acids: Strong acids are completely dissociated (100%). If the acid is
less than 100% dissociated it is termed a weak acid. The kA of these strong acids is very high. (>
20).

Weak acids are partially ionized or dissociated and therefore form few ions. The kA values of
these acids are small in comparison to strong acids. Chemists use a double arrow to indicate their
dissolving in water.

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 views: 1514 posted: 11/20/2010 language: English pages: 6