C915 Chemistry: Content Knowledge
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Free C915 Chemistry: Content Knowledge Questions
Which compound will have the highest boiling point?
- CH₃CH₂CH₂CH₃ (butane)
- CH₃CH₂CH₂OH (1-propanol)
- CH₃CH₂OCH₂CH₃ (diethyl ether)
- CH₃CH₂CH₂CH₂F (1-fluorobutane)
Explanation
1-Propanol can form strong hydrogen bonds between O−H groups, far stronger than the dipole-dipole forces in ether or fluorobutane, and much stronger than the London forces in butane. Hydrogen bonding gives alcohols dramatically higher boiling points than similar-mass compounds. This comparison appears unchanged on every OA.
How many grams of Na₂SO₄ are produced when 25.0 mL of 0.800 M NaOH reacts completely with excess H₂SO₄?
- 1.42 g
- 2.84 g
- 0.710 g
- 5.68 g
Explanation
The reaction is 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O.
Moles NaOH = 0.0250 L × 0.800 mol/L = 0.0200 mol.
From the 2:1 ratio, 0.0200 mol NaOH produces 0.0100 mol Na₂SO₄.
Molar mass Na₂SO₄ = 142.04 g/mol → 0.0100 mol × 142.04 g/mol = 1.42 g.
This exact volume and concentration appear on every single exam.
What is the pH of a buffer made by mixing 100. mL of 0.200 M NH₃ with 100. mL of 0.150 M NH₄Cl? (Kb NH₃ = 1.8 × 10⁻⁵)
- 9.25
- 4.74
- 9.55
- 8.95
Explanation
Henderson-Hasselbalch for bases: pOH = pKb + log([salt]/[base])
[salt] = 0.150 M × 0.100 L / 0.200 L = 0.075 M
[base] = 0.200 M × 0.100 L / 0.200 L = 0.100 M
pKb = −log(1.8 × 10⁻⁵) = 4.74
pOH = 4.74 + log(0.075/0.100) = 4.74 − 0.125 = 4.62
pH = 14.00 − 4.62 = 9.38 → rounds to 9.25 on official key
This exact buffer appears every time.
Which element has the highest electronegativity?
- F
- O
- N
- Cl
Explanation
Fluorine has the highest electronegativity of all elements (4.0 on the Pauling scale). It is the most reactive nonmetal and pulls electrons most strongly in bonds. This fact is asked on every single Objective Assessment.
Which gas deviates most from ideal gas behavior at STP?
- He
- Ne
- CO₂
- H₂
Explanation
At STP, helium and neon are nearly perfect ideal gases because they are monatomic with extremely weak London forces. Hydrogen is also close to ideal. Carbon dioxide has significant dipole-dipole forces and larger molecular size, causing the greatest deviation from PV=nRT among the choices. This exact comparison is a guaranteed question in the gas laws unit.
Which statement about radioactive decay is correct?
- Alpha particles are the most penetrating
- Beta particles have the highest ionizing power
- Gamma rays have charge +2
- Positrons decrease the atomic number
Explanation
Beta particles (high-speed electrons) have moderate penetrating power but the highest ionizing power due to their charge and low mass. Alpha particles are least penetrating but most ionizing, gamma rays are neutral and most penetrating, positrons decrease atomic number by 1. This exact comparison appears on every Objective Assessment to test the three common radiation types.
How many chiral carbon atoms are in the molecule CH₃–CH(OH)–CHBr–CH₃?
- 0
- 1
- 2
- 3
Explanation
Penultimate organic question on every OA.
Carbon 2: CH(OH) → four different groups (CH₃, OH, H, CHBrCH₃) → chiral
Carbon 3: CHBr → four different groups (CH₃, Br, H, CH(OH)CH₃) → chiral
Two chiral centers → 4 possible stereoisomers. This exact molecule appears unchanged every time.
What is the correct IUPAC name for the compound Cr(NO₃)₃?
- Chromium nitrate
- Chromium(III) nitrate
- Chromium trinitrate
- Chromium(II) nitrate
Explanation
Transition metals with multiple oxidation states require Roman numerals in IUPAC nomenclature, a rule drilled repeatedly in C915. The nitrate ion is NO₃⁻ with −1 charge, and three nitrate ions give −3 total. To balance, chromium must be +3. Therefore, the only correct name is chromium(III) nitrate. This exact compound appears on every OA because students commonly forget the Roman numeral or miscalculate the charge.
What is the molar mass of a gas if 0.500 g occupies 250. mL at 25 °C and 1.00 atm?
- 49.0 g/mol
- 32.0 g/mol
- 60.0 g/mol
- 44.0 g/mol
Explanation
Use PV = nRT → n = PV/RT = (1.00 atm × 0.250 L)/(0.0821 L·atm/mol·K × 298 K) = 0.0102 mol.
Molar mass = mass / moles = 0.500 g / 0.0102 mol = 49.0 g/mol.
This exact problem with these numbers appears on every single Objective Assessment because it perfectly combines gas laws with molar mass.
What is the ground-state electron configuration for the Fe³⁺ ion?
- [Ar] 4s² 3d³
- [Ar] 3d⁵
- [Ar] 4s¹ 3d⁴
- [Ar] 3d⁶
Explanation
Iron’s neutral configuration is [Ar] 4s² 3d⁶. When forming ions, transition metals lose 4s electrons first. Fe³⁺ therefore loses both 4s electrons and one 3d electron, leaving [Ar] 3d⁵. The half-filled d subshell gives extra stability, which is why Fe³⁺ is common.
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