JEE Main & Advanced

Shortcut Methods and Tricks:

• Isomer Counting:

• For dibromobutane, count the possible ways of arranging two Br atoms on a 4-carbon chain, considering symmetry: 7 isomers

• For trichlorobenzene, count the possible ways of arranging three Cl atoms on a benzene ring, considering symmetry: 12 isomers

• Boiling Point Estimation:

• Boiling points generally increase with molecular weight, number of branches, and polarity.

• For chloromethane vs bromobenzene: Chloromethane’s lower molecular weight and lack of polarity lead to a lower boiling point (-24°C) than bromobenzene’s higher molecular weight and polarity (156°C).

• Melting Point Estimation:

• Melting points generally increase with intermolecular forces (e.g., van der Waals, hydrogen bonding).

• Iodoform’s high melting point (119°C) is attributed to stronger van der Waals forces due to its larger size and higher molecular weight.

• Density Estimation:

• Density is influenced by molecular weight and intermolecular forces.

• Chloroform’s density (1.48 g/mL) is higher than water (1 g/mL) due to its higher molecular weight and stronger van der Waals forces.

• Solubility Estimation:

• Solubility depends on polarity and intermolecular interactions.

• Carbon tetrachloride’s low solubility in water (0.01 g/100 mL) is because of its nonpolar nature and inability to form hydrogen bonds with water.

• Percentage Composition:

• Calculate the percentage of elements by their relative atomic masses in the compound.

• DDT contains 5 chlorine atoms, each with a mass of 35.5, making up 50% of its molecular mass.

• Teflon contains 2 fluorine atoms, each with a mass of 19, contributing to 76% of its molecular mass.

CBSE Board Exam

Shortcut Methods and Tricks:

• Isomer Counting:

• For dichloroethane, determine the number of possible arrangements of two Cl atoms on a 2-carbon chain: 2 isomers

• Boiling Point Estimation:

• Boiling points increase with molecular weight and branching.

• Bromoethane’s boiling point (38°C) is higher than chloromethane’s (-24°C) due to its higher molecular weight.

• Melting Point Estimation:

• Melting points generally increase with molecular weight and intermolecular forces.

• Iodomethane’s melting point (-66°C) is lower than iodoform’s (119°C) because iodomethane is smaller and has weaker van der Waals forces.

• Density Estimation:

• Density is influenced by molecular weight and intermolecular forces.

• Dichloromethane’s density (1.33 g/mL) is lower than chloroform’s (1.48 g/mL) because it has a lower molecular weight.

• Solubility Estimation:

• Solubility depends on polarity and intermolecular interactions.

• Chloroform’s solubility in water (0.8 g/100 mL) is higher than carbon tetrachloride’s (0.01 g/100 mL) because chloroform can form hydrogen bonds with water.

• Percentage Composition:

• Calculate the percentage of elements based on their relative atomic masses in the compound.

• Ethylene dibromide contains two Br atoms, each with a mass of 80, making up 88% of its molecular mass.

• Iodoform contains one iodine atom with a mass of 127, contributing to 96% of its molecular mass.