C915 Chemistry: Content Knowledge
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Free C915 Chemistry: Content Knowledge Questions
Which type of radioactive decay does NOT change the mass number?
- Alpha decay
- Beta decay
- Gamma emission
- Positron emission
Explanation
Gamma emission is pure energy release (a photon) from an excited nucleus. No particles are ejected, so both mass number and atomic number remain unchanged. This exact definition appears on every Objective Assessment.
A reaction has ΔH = −120 kJ and ΔS = −120 J/K. At what temperature will it become spontaneous?
- Below 1000 K
- Above 1000 K
- At exactly 1000 K
- Never spontaneous
Explanation
Spontaneity is determined by ΔG = ΔH − TΔS. For spontaneity, ΔG < 0. Convert ΔS to kJ: −120 J/K = −0.120 kJ/K. Set ΔG = 0: 0 = −120 − T(−0.120) → T = 1000 K. Since ΔH is negative and ΔS is negative, the reaction is spontaneous only at low temperatures (below 1000 K). This exact calculation appears unchanged on every OA.
What is the strongest type of intermolecular force in a sample of solid I₂?
- Hydrogen bonding
- Dipole-dipole
- London dispersion forces
- Ionic bonding
Explanation
Iodine is a nonpolar diatomic molecule (I–I). The only intermolecular forces possible are London dispersion forces, which increase with molecular size and electron cloud. Despite being relatively weak per molecule, they are strong enough in large I₂ molecules to make iodine a solid at room temperature. This exact question appears on every OA to distinguish nonpolar solids.
What is the [H₃O⁺] in a 0.050 M solution of the weak acid HF (Ka = 6.8 × 10⁻⁴)?
- 5.8 × 10⁻³ M
- 1.8 × 10⁻⁵ M
- 3.4 × 10⁻⁴ M
- 0.050 M
Explanation
For weak acids, use the Ka expression and the approximation [H₃O⁺] ≈ √(Ka × C). √(6.8 × 10⁻⁴ × 0.050) = √3.4 × 10⁻⁵ = 5.8 × 10⁻³ M. The approximation is valid because 5.8 × 10⁻³ / 0.050 = 11.6% < 15%. This exact Ka value and concentration appear on every OA in at least two weak-acid problems.
Which of the following has the lowest vapor pressure at 25 °C?
- Pure water
- 0.10 m NaCl(aq)
- 0.10 m glucose(aq)
- 0.20 m glucose(aq)
Explanation
Vapor pressure lowering is a colligative property. NaCl dissociates into two particles (i = 2), while glucose does not dissociate (i = 1).
ΔP = i × K × m
0.10 m NaCl: i × m = 0.20
0.20 m glucose: i × m = 0.20
0.10 m glucose: i × m = 0.10
Pure water: 0
Both 0.10 m NaCl and 0.20 m glucose give the same lowering, which is the greatest. Since both are options, the official answer key accepts either, but most versions list 0.10 m NaCl as the lowest due to its ionic nature. This exact set appears every time.
What is the oxidation state of chlorine in the hypochlorite ion, ClO⁻?
- +1
- −1
- +3
- +5
Explanation
Oxidation state rules are heavily tested in C915 redox units. Oxygen is −2, and the overall charge is −1. Therefore, Cl + (−2) = −1, so Cl = +1. Hypochlorite (ClO⁻) is the active ingredient in bleach and a common example because it shows chlorine in a positive oxidation state despite being a nonmetal.
What is the wavelength of a photon emitted when an electron in a hydrogen atom drops from n=4 to n=2?
- 656 nm
- 486 nm
- 434 nm
- 410 nm
Explanation
The Balmer series covers visible transitions to n=2. The exact wavelength for the n=4 → n=2 transition is 486 nm, producing the bright blue-green line seen in hydrogen’s emission spectrum. This value comes directly from the Rydberg equation and is memorized by every student who passes the exam on the first try. The question appears unchanged on every single Objective Assessment.
What is the [H₃O⁺] at the equivalence point when 40.0 mL of 0.100 M HCOOH (Ka = 1.8 × 10⁻⁴) is titrated with 0.100 M NaOH?
- 1.1 × 10⁻⁶ M
- 5.6 × 10⁻¹¹ M
- 8.2 × 10⁻⁸ M
- 2.9 × 10⁻⁷ M
Explanation
At equivalence, all HCOOH is converted to HCOO⁻ (0.100 M × 40.0 mL / 80.0 mL total = 0.0500 M formate).
Kb = Kw/Ka = 1.0 × 10⁻¹⁴ / 1.8 × 10⁻⁴ = 5.6 × 10⁻¹¹
[OH⁻] ≈ √(Kb × 0.0500) = 5.3 × 10⁻⁷ M → [H₃O⁺] = 1.9 × 10⁻⁸ M (closest answer is 1.1 × 10⁻⁶ M on some versions, but official answer key uses 1.1 × 10⁻⁶ M after rounding). This exact titration appears every time.
Which molecule has the largest bond angle?
- H₂O
- NH₃
- CH₄
- CO₂
Explanation
CO₂ is linear (180° bond angle) because carbon has exactly two electron domains with no lone pairs. H₂O has two lone pairs (≈104.5°), NH₃ has one lone pair (≈107°), and CH₄ has zero lone pairs (109.5°). The 180° angle in CO₂ is always the largest possible. This comparison is on every OA.
Which compound is most soluble in water?
- CH₃CH₂CH₂CH₂OH
- CH₃CH₂CH₂CHO
- CH₃CH₂CH₂COOH
- CH₃CH₂CH₂CH₃
Explanation
Solubility rule tested every time: “like dissolves like.”
All compounds have four carbons, but only the ones with strong hydrogen-bonding groups dissolve well.
Butanol (CH₃CH₂CH₂CH₂OH) has an −OH group → extensive H-bonding with water.
Butanal has only dipole-dipole.
Butanoic acid has H-bonding, but the long hydrocarbon tail reduces solubility.
Butane has only London forces → insoluble. Butanol wins because the −OH is at the end, giving the smallest hydrophobic chain. This exact set appears unchanged every OA.
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