OAT General Chemistry Principles 4 — Questions and Answers
Question 1: Which of the following has the highest lattice energy?
- NaF
- NaCl
- MgO (Correct answer)
- KBr
Correct answer: MgO
Lattice energy increases with higher ionic charges and smaller ionic radii; MgO (Mg²⁺ and O²⁻) has the highest charge product and small ions.
Question 2: At STP, what volume does 2 moles of an ideal gas occupy?
- 11.2 L
- 22.4 L
- 44.8 L (Correct answer)
- 33.6 L
Correct answer: 44.8 L
One mole of ideal gas occupies 22.4 L at STP, so 2 moles occupy 2 × 22.4 = 44.8 L.
Question 3: Which of the following best describes an endothermic reaction?
- Products have lower energy than reactants
- Heat is released to the surroundings
- ΔH is negative
- ΔH is positive (Correct answer)
Correct answer: ΔH is positive
An endothermic reaction absorbs heat from the surroundings, so the enthalpy change (ΔH) is positive.
Question 4: According to VSEPR theory, the molecular geometry of SF4 is:
- Tetrahedral
- See-saw (Correct answer)
- Square planar
- Trigonal pyramidal
Correct answer: See-saw
SF4 has 4 bonding pairs and 1 lone pair (5 electron groups), giving a see-saw molecular geometry.
Question 5: The half-life of a first-order reaction is 10 minutes. How much of a 100 g sample remains after 30 minutes?
- 50 g
- 25 g
- 12.5 g (Correct answer)
- 6.25 g
Correct answer: 12.5 g
Three half-lives elapse in 30 min; 100 → 50 → 25 → 12.5 g, so 12.5 g remains.
Question 6: Which of the following is an example of a Lewis acid?
- NH3
- H2O
- BF3 (Correct answer)
- OH⁻
Correct answer: BF3
BF3 is an electron-pair acceptor (Lewis acid) because boron has an empty p orbital and an incomplete octet.
Question 7: Hess's Law states that the total enthalpy change for a reaction:
- Depends on the pathway taken
- Is equal to the activation energy
- Is independent of the pathway taken (Correct answer)
- Is always negative
Correct answer: Is independent of the pathway taken
Hess's Law states that ΔH is a state function and is the same regardless of how many steps are used to go from reactants to products.
OAT General Chemistry Principles 4
Which of the following has the highest lattice energy?