UNIT 1.4 SOLUTIONS

LEC:4                                                             Solutions

Mostly of the reaction in laboratories are carried out in the form of solution so we have to study basic of solution, in detail we will study solution in the 12^th standard.

Solutions are homogeneous mixtures of two or more than two components. By homogenous mixture we mean that its composition and properties are uniform throughout the mixture. Generally, the component that is present in the largest quantity is known as solvent. Solvent determines the physical state in which solution exists. One or more components present in the solution other than solvent are called solutes. In this section we shall consider only binary solutions i.e. solution of two componenets. 

Therefore, it is important to understand as how the amount of substance is expressed when it is present in the form of a solution. The concentration of a solution or the amount of substance present in its given volume can be expressed in any of the following ways.

1. Mass per cent or weight per cent (w/w %) or Volume Per cent (v/V%)

2. Mole fraction

3. Molarity

4. Molality

5. Normality

Let us now study four of them in detail, we will Normality study in  12^th standard.

1. Mass per cent or weight per cent (w/w %) or Volume Per cent (v/V%)

The mass percentage of a component of a solution is defined as:

For example, if a solution is described by 10% glucose in water by mass, it means that 10 g of glucose is dissolved in 90 g of water resulting in a 100 g solution

similarly we can define the Volume percent as

For example, 10% ethanol solution in water means that 10 mL of ethanol is dissolved in 90 mL water such that the total volume of the solution is 100 mL.

2. Mole fraction

Commonly used symbol for mole fraction is x and subscript used on the right hand side of x denotes the component. It is defined as:

For example, in a binary mixture, if the number of moles of A and B are nA and nB respectively, the mole fraction of A  & B will be

if there are i component in the solution then there is i mole fraction and sum of all mole fraction is unity.

3. Molarity

Molarity (M) is defined as number of moles of solute dissolved in one litre (or one cubic decimetre) of solution,

For example, 0.25 mol/L (or 0.25 M) solution of NaOH means that 0.25 mol of NaOH has been dissolved in one litre (or one cubic decimetre).

4. Molality

Molality (m) is defined as the number of moles of the solute per kilogram (kg) of the solvent and is expressed as:

For example, 1.00 mol/kg (or 1.00 m) solution of KCl means that 1 mole (74.5 g) of KCl is dissolved in 1 kg of water.

Each method of expressing concentration of the solutions has its own merits and demerits. Mass %, mole fraction and molality are independent of temperature, whereas Molarity is a function of temperature. This is because volume depends on temperature and the mass does not.

Problem: Calculate (a) molality (b) molarity and (c) mole fraction of KI if the density of 20% (mass/mass) aqueous KI is 1.202 g/mL.

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