COMSATS Admission Test Chemistry Syllabus

What is a covalent bond?

A chemical bond formed by the mutual sharing of one or more pairs of electrons between two atoms (typically non-metals).

Distinguish between a polar and a non-polar covalent bond.

Polar covalent: electrons shared unequally due to an electronegativity difference, creating partial charges (e.g., H-Cl). Non-polar covalent: electrons shared equally between atoms of equal/similar electronegativity (e.g., H-H, Cl-Cl).

What is a coordinate (dative) covalent bond?

A covalent bond in which both shared electrons are donated by only one of the two bonded atoms (e.g., the N→B bond in H₃N→BF₃, or the bond in NH₄⁺).

State the octet rule.

Atoms tend to gain, lose, or share electrons to achieve a stable noble-gas configuration of eight electrons in their outermost (valence) shell.

How do single, double, and triple covalent bonds differ in shared electron pairs and bond strength/length?

Single = 1 shared pair, double = 2 pairs, triple = 3 pairs. As bond order increases, bond strength increases and bond length decreases.

What is hybridization?

The mixing of atomic orbitals of similar energy within an atom to form an equal number of new, equivalent hybrid orbitals used for bonding.

What hybridization, geometry, and bond angle correspond to sp orbitals?

sp hybridization: linear geometry, 180° bond angle (2 electron domains), e.g., BeCl₂, C₂H₂.

What hybridization, geometry, and bond angle correspond to sp² orbitals?

sp² hybridization: trigonal planar geometry, 120° bond angle (3 electron domains), e.g., BF₃, C₂H₄.

What hybridization, geometry, and bond angle correspond to sp³ orbitals?

sp³ hybridization: tetrahedral geometry, 109.5° bond angle (4 electron domains), e.g., CH₄.

What hybridizations correspond to trigonal bipyramidal and octahedral geometries?

sp³d = trigonal bipyramidal (90° and 120°), e.g., PCl₅; sp³d² = octahedral (90°), e.g., SF₆.

What does VSEPR theory state?

Valence Shell Electron Pair Repulsion theory: electron pairs (bonding and lone) around a central atom arrange themselves to be as far apart as possible to minimize repulsion, determining molecular geometry.

Why is the H-O-H bond angle in water (104.5°) less than the ideal tetrahedral angle (109.5°)?

Water's oxygen has two lone pairs whose stronger lone pair-lone pair and lone pair-bond pair repulsions compress the bonding angle below 109.5°.

Rank the relative repulsion strengths between lone pairs (lp) and bond pairs (bp).

Lone pair-lone pair > lone pair-bond pair > bond pair-bond pair.

What is the molecular geometry of NH₃ and its approximate bond angle?

Trigonal pyramidal geometry (sp³ with one lone pair), bond angle about 107°.

State the postulates of the kinetic molecular theory of gases.

Gas particles are tiny and far apart (negligible volume); they move randomly in straight lines; collisions are perfectly elastic (no energy loss); there are no intermolecular forces; and average kinetic energy is proportional to absolute temperature.

State Boyle's Law with its equation.

At constant temperature and amount of gas, the volume is inversely proportional to pressure: PV = constant, or P₁V₁ = P₂V₂.

State Charles's Law with its equation.

At constant pressure and amount of gas, the volume is directly proportional to absolute temperature: V/T = constant, or V₁/T₁ = V₂/T₂.

State Avogadro's Law.

At constant temperature and pressure, equal volumes of all gases contain equal numbers of molecules; volume is directly proportional to the number of moles (V ∝ n).

Write the ideal gas equation and define each term.

PV = nRT, where P = pressure, V = volume, n = number of moles, R = universal gas constant (0.0821 L·atm·mol⁻¹·K⁻¹ or 8.314 J·mol⁻¹·K⁻¹), and T = absolute temperature in Kelvin.

State Dalton's Law of Partial Pressures.

The total pressure of a mixture of non-reacting gases equals the sum of the partial pressures of the individual gases: P_total = P₁ + P₂ + P₃ + ...

What is the molar volume of an ideal gas at STP, and what are the STP conditions?

22.414 L per mole at STP (Standard Temperature and Pressure: 0°C / 273.15 K and 1 atm / 101.325 kPa).

State Graham's Law of Diffusion/Effusion.

The rate of diffusion or effusion of a gas is inversely proportional to the square root of its molar mass (or density): rate ∝ 1/√M, so rate₁/rate₂ = √(M₂/M₁).

What is vapor pressure, and how does it relate to boiling point?

Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid. A liquid boils when its vapor pressure equals the external (atmospheric) pressure; lower external pressure lowers the boiling point.

Define surface tension and viscosity of a liquid.

Surface tension is the energy/force at a liquid surface that minimizes surface area (due to inward intermolecular attraction). Viscosity is a liquid's resistance to flow. Both generally increase with stronger intermolecular forces and decrease with rising temperature.

Compare crystalline and amorphous solids.

Crystalline solids have a regular, long-range ordered arrangement of particles, sharp melting points, and are anisotropic (e.g., NaCl, diamond). Amorphous solids lack long-range order, soften over a temperature range, and are isotropic (e.g., glass, rubber).

What is the coordination number in a crystal lattice?

The number of nearest-neighbor particles immediately surrounding a given particle (atom or ion) in the crystal structure.

Define a mole and state the value of Avogadro's number.

A mole is the amount of substance containing as many entities as there are atoms in 12 g of carbon-12. Avogadro's number = 6.022 × 10²³ entities per mole.

How do you calculate the number of moles from a given mass of a substance?

Moles (n) = mass (g) ÷ molar mass (g/mol).

What is the difference between an empirical formula and a molecular formula?

The empirical formula gives the simplest whole-number ratio of atoms in a compound (e.g., CH); the molecular formula gives the actual number of atoms of each element in a molecule (e.g., C₆H₆ for benzene). Molecular formula = (empirical formula)ₙ.

What is a limiting reactant, and how does it relate to theoretical yield and percent yield?

The limiting reactant is the reactant completely consumed first, which limits the amount of product formed. It determines the theoretical (maximum) yield. Percent yield = (actual yield ÷ theoretical yield) × 100%.