CONCENTRATION OF SOLUTIONS

Learning

ü  Solution

ü  Dilute solution

ü  Concentrated solution

ü  Saturated solution

ü  Mass percentage(W/W)

ü  Volume Percentage(V/V)

ü  Mass by volume percentage

ü  Molarity (M)

ü  Molality (m)

ü  Normality(N)

ü  Formality (F)

ü  Mole fraction

ü  Parts per million (ppm)

ü  Strength (S)

 What is solution?

We have experienced with so many large numbers of mixtures in daily life, the air we breathe,the water we drink, all of the fluids in our bodies, the liquids we intake(tea,coffee,soda water,sarbat, wine,Chhas) and the different chemical reactions mixture usesin Laboratory.Scientificallythese mixturesare called solution.

 

 

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Definition

 

Solution is a homogeneous mixture of one or more solutes that have been dissolved in a solvent.

Examples-sea water,Air mixture,sugar solution,Vinegar,Acetone,formalin,soft drink,blood,tea,coffee.

The solute is the substance that needs to dissolve, and the solvent is the thing that needs to dissolve it. Generally, the solvent presence is large excess amount than the solute in a solution. A solute may be a solid, liquid, or gas. Similarly, solvents may be ofgases, liquids, or solids. The solvent determines the phase of the solution as it is present in higher amounts than the solute.

The composition content ofa solution can be expressed by the term Concentration. Concentration is defined as the amount of the solute present in a given amount of solvent or solution.

 Concentration can be expressed quantitatively or qualitatively.  We can say qualitatively   the solution may be diluted orconcentrated solution.

 

A solution is said to be dilute solution if it contains small amount of dissolved solute whereas a concentrated solution contains relatively more amount of dissolved solute.But, whena concentration of a solute maximum in a solutionor no more solute will be dissolved such solution is called Saturated solution.

 In real life theseexpression are given so confusion,practically not possible the exact concentration of the solution.So as compare toqualitative expression of concentration quantitative expression is essential.

The expression of concentration in quantitatively can be expressed in various ways as follows.

1.Mass percentage(W/W)

The mass percentage of a solute in a solution is defined as-

Mass % of a solute =(Mass of the Solute in solution/Total mass of the solution) X 100

 

Example- 5 %W/W glucose solution in water, it can be prepare 5gm of glucose dissolved in 95gmof water resulting in 100gm of solution. Generally this concentration expression is carried out in Chemical industry.

2.Volume Percentage(V/V)

The volume percentage can be defined as

Volume % of a solute =(Volume of the Solute /Total volume of the solution) X100

Example,

 

10% V/V ethanol solution in water ,10 ml of ethanol dissolve in 90 ml of water so that total volume of solution will be 100 ml.

Mostly solutions containing liquid are expressed in such unit.

3. Mass by volume percentage(W/V)

It is defined as the mass of the solute dissolved in the 100 ml of the solution

Example-0.9% W/V NaCl solution,it contains 0.9 gm of NaCl dissolved in water up tovolume make up 100ml and preparing 100 ml of Solution.

Generally, this expression of concentration is widely used in medicine and pharmacy. This concentration expression is easier to handle, as the volume of solvent and solution is easier to measure than the weight.

4. Molarity (M)

It is most widely used unit for concentration expression of the solution.It may be defined as the number of moles of the solute present per litre of the solution.Molarity is denoted as M and its unit is mol L-1.

Molarity may be expressed in various ways as M/10 (0.1 M) Decimolar, M/2 (0.5 M) Semimolar, M/5 (0.2 M) Pentimolar, M/100 (0.01 M) Centimolar, M/1000 (0.001 M) millimolar.

For calculating the molarity of any one solution, if other solution molarity and volume of two solution known,it can be calculated by the formula

 

M1V1 = M2V2 

Example- 1 M NaOH means 1mol of NaOH present in 1 litre of the solution,So here 40 gm of NaOH (Mol Mass=40)is present in 1litre of solution.

Molarity changes with temperature due to changes of volume or concentration of solution with change in temperature.

5. Molality (m)

It is defined as the number of moles of the solute present in 1000gm (1Kg) of the solvents.

 

Molarity (M) =(No. of moles of Solute / mass of solvent in Kg)

Molality does not change with temperature,as the mass does not change by temperature.

 

Molality does not change with temperature,as the mass does not change by temperature.

6. Normality(N)

 

Normality is defined as the no of gram equivalents of the solute present per litre of the solution.

Example-Preparation of 1 N NaOH solution,

Eqv. weight of NaOH = mol. Mass of NaOH / Acidity = 40 / 1 = 40 so, 40gm of NaOH is required to dissolve in 1litre of solution.

For calculating the normality of any one solution, if other solution normality and volume of two solution known,it can be calculated by the formula

N1V1 = N2V2

Relationship between Normality (N)and Molarity (M)

N X Equivalent Weight = Molar Mass X Molarity

N = Number of H+ or OH– ion X Molarity

N= Valency X Molarity

A solution having normality equal to unity is called a normal solution(1N)

If a normality of solution is One it is called Normal solution(1N),Similarly = (0.1 N or N/10) =Deci-normal, = (0.5 N or N/2)= Semi-normal

Normality also changes with temperature.

7.Formality (F)

It is defined as the number of formula mass in gram present in one litre of a solution.

When the formula mass of a solute is equal to its molar mass,the formality is equal to molarity.Formality is depend upon the temperature. So,its concentration changes also it is only applicable for ionic compounds.

8.Mole fraction

 

It is the ratio of the numbers of moles of solute or solvent to the total number of moles in the solution

Where n1=no. of moles of solute,n2= no. moles of solvent

Numbers of mole = weight of the substance / molecular mass of the substance (W/M)

The sum of the mole fraction of solute and solvent is always unity.

XA+ XB =1

This concentration expression used mostly in solution preparation,Molefraction is not depending upon temperature,so it does not change in changes of temperature.

9.Parts per million (ppm)

 

It is the number of parts of solute component present per million partsof the solution

This concentration express is used in a solution which contain very trace amount of solute example-impurities present in chemicals andpharmaceuticals,Air,heavy metals in our foods etc

10.Strength (S)

Strength of a solution is expressed as the amount of solute in grams dissolvedper litre of the solution.

Strength (S) = Amount of solute in gm / Volume of solution in litre

 

The unit of strength is gram per litre