Rationale: Ammonia (\(NH_3\)) has 5 valence electrons from nitrogen and 3 from the hydrogens = 8 total valence electrons (4 electron pairs). Nitrogen forms three single bonds to hydrogen and retains one lone pair. The four electron regions create a tetrahedral electron geometry, but the physical shape of the molecule is trigonal pyramidal.
Q8 Correct Answer: B hydrogen bonds
Rationale: Hydrogen bonding is a strong dipole-dipole attraction that occurs when H is bonded directly to highly electronegative F, O, or N. Since water (\(H_2O\)) contains direct O-H bonds, its primary intermolecular force is hydrogen bonding.
Q9 Correct Answer: C nonpolar molecule with polar bonds
Rationale: Carbon tetrafluoride (\(CF_4\)) has polar C-F bonds due to the high electronegativity of F. However, because the molecule has a highly symmetric tetrahedral geometry, the individual dipoles point symmetrically in opposite directions and cancel, leaving the molecule nonpolar.
Q10 Correct Answer: A HBr
Rationale: HBr consists of two different nonmetal atoms with different electronegativities, resulting in an unequal sharing of electrons and a polar covalent bond. Br₂ and O₂ are nonpolar covalent (shared equally). KBr and MgS are ionic.
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Unit 2: The Mole & Molar Mass Calculations Solutions Questions 11-14
Q11 Correct Answer: A 1.81 × 10²⁴ carbon atoms
Step-by-Step Dimensional Analysis:
Determine carbon moles: Each mole of glucose (C₆H₁₂O₆) contains 6 moles of C atoms.
moles of C = \(0.500 \text{ mol glucose} \times \frac{6 \text{ mol C}}{1 \text{ mol glucose}} = 3.00 \text{ moles of C}\).
Convert to atoms using Avogadro's number:
atoms of C = \(3.00 \text{ mol C} \times 6.022 \times 10^{23} \text{ atoms/mol} = 1.81 \times 10^{24} \text{ carbon atoms}\).
Q12 Correct Answer: A 110.98 g/mole
Molar Mass Calculation Process:
Identify element atomic weights: Calcium (Ca) = 40.08 g/mol, Chlorine (Cl) = 35.45 g/mol.
• Mass of 1 mol of Ca = 1 × 40.08 g/mol = 40.08 g/mol
• Mass of 2 mol of Cl = 2 × 35.45 g/mol = 70.90 g/mol
• Total Molar Mass of CaCl₂ = 40.08 + 70.90 = 110.98 g/mol
Q13 Correct Answer: D 180.16 g/mole
Aspirin (C₉H₈O₄) Molar Mass breakdown:
Atomic weights: Carbon = 12.01 g/mol, Hydrogen = 1.01 g/mol, Oxygen = 16.00 g/mol.
• Carbon (C): 9 atoms × 12.01 g/mol = 108.09 g/mol
• Hydrogen (H): 8 atoms × 1.01 g/mol = 8.08 g/mol
• Oxygen (O): 4 atoms × 16.00 g/mol = 64.00 g/mol
• Molar Mass = 108.09 + 8.08 + 64.00 = 180.17 g/mol (rounded to 180.16)
Q14 Correct Answer: B 120.2 g
Converting Moles to Mass:
To convert moles to grams, multiply the given moles by the atomic mass of Calcium (40.08 g/mol):
Mass = \(3.00 \text{ mol Ca} \times 40.08 \text{ g/mol} = 120.24 \text{ grams} \approx 120.2 \text{ g}\).
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Unit 3: Reaction Classifications & Balancing Solutions Questions 15-18
Q15 Correct Answer: A 1, 5, 3, 4
Balancing Propane Combustion: For C₃H₈ + O₂ ➔ CO₂ + H₂O. (1) Carbon: 3 on left ➔ place 3 on CO₂. (2) Hydrogen: 8 on left ➔ place 4 on H₂O (4 × 2 = 8). (3) Oxygen: count on right: (3 × 2) + (4 × 1) = 10 oxygens. Place 5 on O₂ (5 × 2 = 10). Balanced: 1, 5, 3, 4.
Q16 Correct Answer: C 3
Balancing Single Replacement: Al + HCl ➔ AlCl₃ + H₂. The common multiple of 2 (from H₂) and 3 (from AlCl₃) is 6. Place 6 on HCl. This requires 2 AlCl₃ and 3 H₂ to balance Cl and H. Finally, place 2 on Al. Balanced: 2Al + 6HCl ➔ 2AlCl₃ + 3H₂. The coefficient for H₂ is 3.
Q17 Correct Answer: C Decomposition
Reaction Classification: A single reactant (CaCO₃) splits into multiple products (CaO + CO₂). This matches the classic pattern of a decomposition reaction (AB ➔ A + B).
Q18 Correct Answer: D Magnesium
Redox Analysis: In Mg + 2HCl ➔ MgCl₂ + H₂. Solid Mg metal has an oxidation state of 0. In MgCl₂, magnesium has a +2 charge (lost 2 electrons). Hydrogen in HCl is +1 and is reduced to 0 in H₂. Because Mg loses electrons, it is oxidized (OIL: Oxidation Is Loss).
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Unit 3: Stoichiometry & Thermodynamics Solutions Questions 19-21
Q19 Correct Answer: D 2 moles NH₃ / 3 moles H₂
Mole Ratio Conversion Factor: We convert from moles of \(H_2\) to moles of \(NH_3\). The starting unit (mol \(H_2\)) must be in the denominator to cancel, and our target unit (mol \(NH_3\)) in the numerator. Based on the coefficients of the balanced equation, we get: \(\frac{2 \text{ mol } NH_3}{3 \text{ mol } H_2}\).
Q20 Correct Answer: D 4.03 g
Three-Step Stoichiometry Process:
Q21 Correct Answer: D An exothermic reaction
Thermodynamics: Because heat energy (+92.2 kJ) is listed explicitly as a product on the right side of the chemical equation, heat is being released to the surrounding environment during the process, which defines an exothermic reaction.
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Unit 4: Polarity, Electrolytes & Solubility Solutions Questions 22-25
Q22 Correct Answer: C hexane is nonpolar and water is polar
"Like Dissolves Like": Solutes dissolve in solvents of matching polarity. Water is highly polar, while hexane (C₆H₁₄) is a symmetric nonpolar hydrocarbon with zero net dipole. Mismatched polarities prevent them from dissolving in each other.
Q23 Correct Answer: B weak electrolyte
Electrolyte Classifications: Strong electrolytes dissociate 100% to ions. Nonelectrolytes dissolve completely as whole molecules. Weak electrolytes dissociate partially, leaving both molecules and ions in equilibrium in the solution.
Q24 Correct Answer: E insoluble silver halide exception
Solubility Rules: Halide salts (Cl⁻, Br⁻, I⁻) are generally soluble, except when paired with silver (Ag⁺), lead (Pb²⁺), or mercury (Hg₂²⁺). Silver chloride (AgCl) is a classic insoluble exception and forms a white precipitate.
Q25 Correct Answer: B increases as the gas pressure above the liquid increases
Henry's Law: The solubility of a gas in a liquid is directly proportional to the gas pressure above the liquid. Higher partial pressure of the gas forces more gas molecules to dissolve.
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Unit 4: Concentration & Molarity Solutions Questions 26-30
Q26 Correct Answer: B 20.0%
Mass % = [g solute / total g solution] × 100%
Mass % = [25.0 g / (25.0 g KCl + 100.0 g water)] × 100%
Mass % = [25.0 g / 125.0 g] × 100% = 20.0%.
Q27 Correct Answer: A 12.5 mL
Volume % = [mL solute / total mL solution] × 100%
mL acetic acid = 250 mL solution × (5.0% / 100) = 250 × 0.050 = 12.5 mL.
Q28 Correct Answer: E 300 mL
Mass/Volume % = [g solute / mL solution] × 100%
mL solution = [45 g / 15%] × 100 = 300 mL (which can be written as 3.0 × 10² mL).
Q29 Correct Answer: B 0.50 M
Q30 Correct Answer: A 0.50 L
Molarity = moles / Liters ➔ Volume (L) = moles / Molarity.
Volume (L) = 1.0 mole HCl / 2.0 M = 0.50 Liters of solution (or 500 mL).
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