Chemistry CONCENTRATION IN TERM OF MOLE FRACTION , MOLARITY , MOLALITY AND NORMALITY

Concentation terms :

Many chemical reactions occur in the solution state and hence stoichiometric problems may sometimes involve different concentration terms of solutions to give an idea of amount of solute and solvent present in the solution. There are various ways to represent concentration of the solution as mentioned below.

In a solution the designation of solute and solvent is often a matter of convenience, however many a times the one present in smaller quantity is termed as solute. Also, a solution may have more than one solute but solvent cannot be more than one.

All the concentration terms given below are applicable only when there is a homogenous solution or when solute completely dissolves in the solvent.

The following concentrations terms are used to expresse the concentration of a solution. These are as follows

a.) Molarity `(M)`
b.) Molality `(m)`
c.) Mole fraction `(X)`
d.) `%` calculation
e.) Normality `(N)`
f.) ppm

Please remember that all these concentration terms are related to one another. By knowing one concentration term you can also find the other concentration terms. Let us discuss all of them one by one.

Molarity :

The number of moles of a solute dissolved in `1 L` (`1 000` `ml`) of the solution is known as the molarity of the solution.

`text(Molarity of solution) = text(number of moles)/text(volume of solution in litre)`

Let a solution is prepared by dissolving `w` gm of solute of mol. wt. `M` in `V` ml water.

`therefore` Number of moles of solute dissolved = `w/M`

`therefore` V ml water have `w/M` mole of solute

`:.` 1000 m water have `(wxx 1000)/(Mxx V(ml))`

`:.` Molarity (`M`) = `(wxx 1000)/(Mxx V(ml))`

Some other relations may also be useful.

Number of millimoles = `text(mass of solute)/text(Mol.wt. of solute) xx1000` = `text(Molarity of solution) xx V quad text(in mL)`

Molarity= `text(mass of solute)/text(Total volume of solution in mL)`

If a particular solution having volume `V_1` and molarity = `M_1` is diluted to `V_2` mL then

`M_1 V_1 =M_2 V_2`, `M_2=>` Resultant molarity

If a solution having volume `V_1`, and molarity = `M_1` is mixed with another solution of same solute having volume `V_2` ml and molarity `M_2`

`M_R = (M_1V_1 + M_2V_2)/(V_1 + V_2)`

Molarity is a unit that depends upon temperature. It varies inversely with temperature. Mathematically: Molarity decreases as temperature increases.

`text(Molarity) prop 1/text(Temperature) prop 1/text(volume)`

Molality `(m)` :

The molality is the number of moles of solute present in one `Kg` of solvent

`m = (w_(solute) xx 1000)/(text(Molecular Mass of solute) xx w_(solvent)(gm))`

Molality is independent of temperature changes.

Mole Fraction `(X)`

The ratio of number of moles of the solute or solvent present in the solution and the total number of moles present in the solution is known as the mole fraction of substance concerned.

Let number of moles of solute in solution = `n`

Number of moles of solvent in solution = `N`

`:.`Mole fraction of solute(`X_1`) = `n/(n+N)`

`:.` Mole fraction of solvent (`X_2`) = `N/(n+N)`

Also `X_1 + X_2` = `1`

Mole fraction is a pure number. It will remain independent of temperature changes.

Percentage Concentration :

The concentration of a solution may also be expressed in terms of percentage in the following way.

a.) `%` weight by weight `(w//w)` :

It is given as mass of solute present in per `100` `gm` of solution.

`% (w//w)=text(mass of solute in gm)/text(mass of solution in gm) xx 100`

b.) `%` weight by volume `(w//V)` :

It is given as mass of solute present in per `100` mL of solution.

``% (w//V)=text(mass of solute in gm)/text(volume of solution in mL) xx 100`

c.) `%` Volume by Volume `(V//V)` :

It is given as volume of solute present in per `100` mL solution.

`% (V//V)=text(volume of solution in mL)/text(volume of solution in mL) xx 100`

ppm

Parts per million (ppm) - Amount of solute (in g) with `10^6` g solvent.

Parts per billion (ppb) - Amount of solute (in g ) with `10^9` g solvent.

Normality:

Normality could be defined as the number of gram equivalents of a solute present per litre (`dm^3`) of the solution at any given temperature and it is expressed as `N`.

Normality = `text(Equivalents of solute)/text(volume of solution in litre)`

The Normality of the solution can also be expressed in terms of mass and equivalent mass

Normality = `text(Mass of the solute)/(text(Equivalent mass of the solute) xx text(Volume of the solution in litres))`

Measurements in normality can change with the change in temperature because solutions expand or contract accordingly.