Ncert Solutions Chemistry Class 12th

(a) Molality, also called molal concentration, is a measure of the concentration of a solute in a solution in terms of amount of substance in a specified amount of mass of the solvent. Molar mass of KI = 39 + 127 = 166 g/mol.
20% aqueous solution of KI means 200 g of KI is present in 1000 g of solution. Therefore,  

Molality = Moles of KI / Mass of Water in kg

= (200/166) / (0.8) = 1.506 m

(b) Molarity is the concentration of a solution expressed as the number of moles of 
solute per litre of solution.
Given,
Density of the solution = 1.202 g/mL
Volume of 100 g solution = mass/ density
 = 100/1.202 
 = 83.19 mL
Therefore, molarity = 20/166 mol/ 83.19 * 10^-3 L 
 = 1.45 M
∴ Molarity of KI = 1.45M

(c) Molar mass of KI = 39 + 127 = 166 g/mol.
Moles of KI = 20/166 = 0.12 mol
Moles of water = 80/18 = 4.44 mol

Therefore,
Mole fraction of KI = Moles of KI / Moles of KI+ Moles of Water
 = 0.12 / 0.12+ 4.44
 = 0.0263
∴ Mole fraction of KI = 0.0263

16.28 

The commonly used soaps are the sodium and potassium salts of higher fatty acids. They are soluble in water and give good lather to it. On the other hand, calcium and magnesium salts of higher fatty acids (calcium and magnesium soaps) are insoluble in water and do not produce lather. Hard water contains Ca²⁺and Mg²⁺ ions. When a sodium or potassium soap is added to hard water, it gets converted into an insoluble calcium or magnesium soap as shown ahead.

Due to the conversion of sodium or potassium soaps into calcium or magnesium soaps in the presence of hard water, the common soaps are unable to emulsify the greasy dirt and clean the dirty object. They do not produce lather with hard water. The calcium and magnesium soaps thus produced appear on the surface as insoluble sticky grey scum. This involves a waste of the common soap and the fabrics being washed get discoloured and hardened. This is why the commonly used sodium and potassium soaps cannot be used as cleansing agents with hard water.

16.27 

It has been found that the detergents having a great deal of branching in the hydrocarbon tail are not biodegradable and cause pollution in rivers and waterways. This is because the presence of side chains in the hydrocarbon tail stops bacteria from attacking and breaking the chains.

This results in slow degradation of detergent molecules leading to their accumulation in water ways. Alkylbenzene sulphonates having branched chain alkyl groups possess poor biodegradability and cause pollution in rivers and water Such detergents are termed as non-biodegradable or hard detergents.

The alkyl sulphates do not possess branching and are therefore biodegradable. They do not cause pollution. Alkyl benzene sulphonates in which the phenyl group is either attached to the end of a long chain alkyl group are also biodegradable and do not cause pollution. These are termed as soft detergents.

Nowadays, efforts are being made to keep the amount of branching in the detergent molecule to a minimum. Since unbranched chains are more prone to attack by bacteria, the detergents having no branching or minimum branching are easily biodegraded and pollution is prevented. Example: Sodium-4- (1,3,5,7-Tetra Methyl Octyl) Benzene Sulphonate

16.26 

(i) Cationic detergents- Cationic detergents are quaternary ammonium salts [chlorides, bromides, acetates, etc.] having long chain alkyl groups. The cationic detergents are more expensive than anionic detergents and hence they find only limited use. However, they possess germicidal properties and are used quite extensively as germicides. The examples of cationic detergents are as follows:

(ii) Anionic detergents- A detergent is said to be anionic when the large part of its molecule is an anion and is involved in the cleansing action. The anionic detergents are also effective in slightly acidic In slightly acidic solutions, they form an alkyl hydrogen sulphate which is a soluble material. In contrast, soaps react with acidic solutions to form insoluble fatty acids.The examples of anionic detergents are as follows:

(iii) Non-ionic detergents- These do not contain any ion in their constitution. They are the esters of high molecular mass formed by the reaction between polyethylene glycol and stearic acid. Some non-ionic detergents are used as dishwashing The example of a non-ionic detergent is as follows:

16.25 

Synthetic detergents are superior cleansing agents as compared to soaps. This is due to the following reasons.

1. Detergents are soluble even in hard water. This is because of calcium and magnesium ions present in hard water form soluble salts with detergents. Hence, detergents can be used both in soft as well as in hard water. On the other hand, soaps form insoluble salts with calcium and magnesium ions and cannot be used in hard water.

2. The aqueous solutions of detergents are usually neutral. Therefore, they do not damage delicate fabrics and can be used for washing almost all types of On the other hand, aqueous solutions of soap are alkaline and damage delicate fabrics. Therefore, soaps cannot be used for washing delicate fabrics.