Ncert Books Class 12 Chemistry Chapter 2 Free Download
FAQs on Ncert Books Class 12 Chemistry Chapter 2 Free Download
1. What types of numerical problems are considered most important from Chapter 2, Solutions, for the CBSE Class 12 Board Exam 2025-26?
For the 2025-26 board exams, you should focus on numerical problems related to colligative properties. The most frequently asked questions involve:
Calculating molar mass from properties like depression in freezing point (ΔTf) and elevation in boiling point (ΔTb).
Problems involving osmotic pressure (Π) to determine the molar mass of solutes.
Calculating the van't Hoff factor (i) and using it to find the degree of dissociation or association of a solute.
Interconversion between concentration terms like molarity, molality, and mole fraction.
2. What are the expected 3-mark questions on Raoult's Law and its applications to ideal and non-ideal solutions?
A common 3-mark question involves stating Raoult's Law and differentiating between ideal and non-ideal solutions. You should be prepared to explain:
Ideal solutions: Obey Raoult's Law, ΔH_mix = 0, ΔV_mix = 0.
Non-ideal solutions showing positive deviation: A-B interactions are weaker than A-A and B-B interactions (e.g., ethanol and acetone). Vapour pressure is higher than expected.
Non-ideal solutions showing negative deviation: A-B interactions are stronger than A-A and B-B interactions (e.g., chloroform and acetone). Vapour pressure is lower than expected.
You may also be asked to draw and label the vapour pressure graphs for these deviations.
3. How can a student solve 5-mark questions related to the van't Hoff factor and abnormal molecular mass in the Solutions chapter?
To master 5-mark questions on the van't Hoff factor (i), follow these steps:
First, identify if the solute is an electrolyte (undergoes dissociation/association) or a non-electrolyte.
Use the given data (e.g., freezing point depression) to calculate the observed colligative property.
Calculate the normal molar mass from the chemical formula and then the theoretical colligative property.
Determine the van't Hoff factor using the formula: i = (Observed Colligative Property) / (Calculated Colligative Property).
Finally, use the value of 'i' to calculate the degree of dissociation (α) or association (α) as required by the question. This comprehensive approach is crucial for scoring full marks.
4. Which colligative properties are most important for the Class 12 Chemistry board exam, and how are they defined?
All four colligative properties are important, but questions are most frequently asked from depression in freezing point and osmotic pressure. Colligative properties are properties of solutions that depend on the number of solute particles, not on their identity. The four important properties are:
Relative Lowering of Vapour Pressure: The ratio of lowering of vapour pressure to the vapour pressure of the pure solvent.
Elevation in Boiling Point (ΔTb): The increase in the boiling point of a solvent upon addition of a non-volatile solute.
Depression in Freezing Point (ΔTf): The decrease in the freezing point of a solvent upon addition of a non-volatile solute.
Osmotic Pressure (Π): The external pressure required to prevent osmosis when a solution is separated from its pure solvent by a semipermeable membrane.
5. Why do some solutions exhibit positive and negative deviations from Raoult's Law? What is the conceptual basis for this important exam topic?
The conceptual basis for deviations from Raoult's Law lies in the intermolecular forces of attraction between solute (A) and solvent (B) particles. This is a key concept for board exams:
Positive Deviation: Occurs when the new A-B interactions are weaker than the original A-A and B-B interactions. This makes it easier for molecules to escape into the vapour phase, increasing the total vapour pressure above the ideal value.
Negative Deviation: Occurs when the new A-B interactions are stronger than the original A-A and B-B interactions (e.g., due to hydrogen bonding). This makes it harder for molecules to escape, decreasing the total vapour pressure below the ideal value.
6. From an exam perspective, how should one explain the contrasting effects of temperature on the solubility of solids versus gases in a liquid?
This is a frequently asked conceptual question. The explanation should be based on Le Chatelier's Principle:
Solubility of Solids: The dissolution of most solids in a liquid is an endothermic process (ΔH_sol > 0). According to Le Chatelier's Principle, increasing the temperature will favour the forward (dissolution) process, thus increasing the solubility.
Solubility of Gases: The dissolution of a gas in a liquid is an exothermic process (ΔH_sol < 0). Therefore, increasing the temperature favours the backward process (gas escaping the solution), which decreases the solubility. This is why cold drinks release dissolved CO₂ gas and go flat when left at room temperature.
7. Beyond calculations, what are the practical applications of colligative properties that could be asked as a HOTS (Higher Order Thinking Skills) question?
HOTS questions test your ability to apply concepts. Important applications include:
Depression in Freezing Point: Using salts like NaCl or CaCl₂ to clear snow from roads in cold countries, or using ethylene glycol as an antifreeze in car radiators.
Osmotic Pressure: The preservation of meat and fruits by salting or adding sugar, which causes bacteria to lose water through osmosis and die. A crucial application is reverse osmosis (RO) for the desalination of seawater to get potable water.
Elevation in Boiling Point: Using ethylene glycol in car radiators also raises the boiling point of the coolant, preventing it from boiling over in summer.
8. Why is molality often considered a more reliable unit of concentration than molarity for important scientific experiments?
This is a critical concept for both theory and viva questions. Molality (m) is considered more reliable than Molarity (M) because it is temperature-independent. The reason is:
Molarity is defined as moles of solute per litre of solution. The volume of the solution can expand or contract with changes in temperature.
Molality is defined as moles of solute per kilogram of solvent. The mass of the solvent does not change with temperature.
Therefore, for experiments where temperature varies, using molality ensures the concentration remains constant and the results are accurate.
