NYSTCE Chemistry (007) — Questions and Answers
Question 1: Which of the following electron configurations represents a ground-state atom in its most stable form?
- 1s² 2s² 2p⁶ 3s¹
- 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² (Correct answer)
- 1s² 2s² 2p⁷
- 1s² 2s¹ 2p⁶ 3s²
Correct answer: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s²
1s² 2s² 2p⁶ 3s² 3p⁶ 4s² is the ground-state configuration of calcium (Z=20), following the Aufbau principle, Pauli exclusion principle, and Hund's rule. Option C is impossible (2p holds max 6 electrons); options A and D violate fill-order rules.
Question 2: A reaction has ΔH = –120 kJ/mol and ΔS = –200 J/(mol·K). At what temperature (in Kelvin) will the reaction transition from spontaneous to non-spontaneous?
- 600 K (Correct answer)
- 60 K
- 0.6 K
- 1200 K
Correct answer: 600 K
Using ΔG = ΔH – TΔS = 0 at the transition point: T = ΔH/ΔS = 120,000 J/mol ÷ 200 J/(mol·K) = 600 K. Below 600 K the reaction is spontaneous (ΔG < 0); above 600 K it becomes non-spontaneous.
Question 3: In a titration, 25.0 mL of 0.200 M NaOH is required to neutralize 20.0 mL of an unknown monoprotic acid. What is the molarity of the acid?
- 0.160 M
- 0.250 M (Correct answer)
- 0.400 M
- 0.100 M
Correct answer: 0.250 M
Moles of NaOH = 0.0250 L × 0.200 mol/L = 0.00500 mol. For a monoprotic acid, moles of acid = moles of base = 0.00500 mol. Molarity of acid = 0.00500 mol ÷ 0.0200 L = 0.250 M.
Question 4: Which statement correctly describes the behavior of a catalyst in a chemical reaction?
- It increases the activation energy, making the reaction proceed more slowly
- It is consumed in the reaction and must be replenished
- It lowers the activation energy and is not consumed in the overall reaction (Correct answer)
- It shifts the equilibrium position to favor products
Correct answer: It lowers the activation energy and is not consumed in the overall reaction
A catalyst provides an alternative reaction pathway with lower activation energy, increasing reaction rate without being consumed. It does not change the thermodynamic equilibrium position (ΔG or Keq) — it only affects the kinetics.
Question 5: An unknown aqueous solution turns litmus paper red and reacts with zinc metal to produce hydrogen gas. Which of the following could be the solute?
- NaCl
- HCl (Correct answer)
- NaOH
- CH₃OH
Correct answer: HCl
HCl (hydrochloric acid) turns litmus red (acidic pH) and reacts with active metals like zinc (Zn + 2HCl → ZnCl₂ + H₂). NaCl is neutral, NaOH is basic, and methanol does not react with zinc under standard conditions.
Question 6: Which intermolecular force is primarily responsible for water's unusually high boiling point relative to other molecules of similar molar mass?
- London dispersion forces
- Dipole-dipole interactions
- Hydrogen bonding (Correct answer)
- Ion-dipole interactions
Correct answer: Hydrogen bonding
Water molecules form extensive hydrogen bonds (O–H···O) due to the highly electronegative oxygen and the small, highly polarizing hydrogen. These relatively strong intermolecular forces require significantly more thermal energy to overcome, elevating the boiling point well above comparable non-hydrogen-bonding molecules like H₂S.
NYSTCE Chemistry (007)
Which of the following electron configurations represents a ground-state atom in its most stable form?