Free practice final
General Chemistry 1 practice final (free, with answers)
A General Chemistry 1 final usually tests stoichiometry and solutions, gas laws, thermochemistry, atomic structure and periodic trends, and bonding and molecular shape. Work each problem on paper, with units, before you open the answer, and read the explanation even when you got it right.
Topics
- Stoichiometry
- Solutions and Titrations
- Gases
- Thermochemistry
- Atomic Structure
- Periodic Trends
- Bonding and Molecular Shape
- Intermolecular Forces
15 questions
Try each one before you open its answer.
1. For N₂(g) + 3H₂(g) → 2NH₃(g), what is the maximum mass of NH₃ that can form from 28.0 g of N₂ and 9.00 g of H₂? (Molar masses: N₂ 28.02, H₂ 2.016, NH₃ 17.03 g/mol.)
- A 17.0 g
- B 37.0 g
- C 34.0 g
- D 50.7 g
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Answer: C. 34.0 g
28.0 g N₂ is 0.999 mol, which needs 3.00 mol H₂; 9.00 g H₂ is 4.46 mol, so H₂ is in excess and N₂ is limiting. 0.999 mol N₂ gives 2 × 0.999 = 2.00 mol NH₃, or 34.0 g (50.7 g is what the H₂ alone could make).
2. A hydrocarbon is 85.6% carbon and 14.4% hydrogen by mass, and its molar mass is 56.1 g/mol. What is its molecular formula? (C = 12.01, H = 1.008 g/mol.)
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Answer: C₄H₈
In 100 g there are 85.6/12.01 = 7.13 mol C and 14.4/1.008 = 14.3 mol H, a 1 : 2 ratio, so the empirical formula is CH₂ (14.03 g/mol). Since 56.1/14.03 = 4, the molecular formula is C₄H₈.
3. A 25.0 mL sample of sulfuric acid, H₂SO₄, needs 32.4 mL of 0.150 M NaOH to reach the equivalence point. What is the molarity of the H₂SO₄?
- A 0.0972 M
- B 0.194 M
- C 0.116 M
- D 0.0579 M
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Answer: A. 0.0972 M
Moles of NaOH = 0.150 M × 0.0324 L = 4.86 × 10⁻³ mol. H₂SO₄ is diprotic (H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O), so moles of acid = 2.43 × 10⁻³ mol, and 2.43 × 10⁻³ mol / 0.0250 L = 0.0972 M.
4. What volume of 12.0 M HCl is needed to prepare 500.0 mL of 0.800 M HCl? Give the answer to three significant figures.
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Answer: 33.3 mL
Moles of solute stay the same on dilution, so M₁V₁ = M₂V₂. V₁ = (0.800 M × 500.0 mL)/12.0 M = 33.3 mL, which is then diluted with water to 500.0 mL.
5. An unknown gas has a density of 1.96 g/L at 0 °C and 1.00 atm. Which of these gases could it be? (R = 0.08206 L·atm/(mol·K).)
- A O₂
- B CO₂
- C SO₂
- D Ar
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Answer: B. CO₂
From PV = nRT, molar mass = dRT/P = (1.96 g/L)(0.08206)(273.15 K)/(1.00 atm) ≈ 43.9 g/mol. That matches CO₂ (44.01 g/mol); O₂ is 32.00, SO₂ is 64.07 and Ar is 39.95 g/mol.
6. A container holds 2.00 g of helium and 8.00 g of oxygen gas (O₂) at a total pressure of 1.50 atm. What is the partial pressure of O₂? (He = 4.00 g/mol, O₂ = 32.00 g/mol.)
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Answer: 0.500 atm
There are 0.500 mol He and 0.250 mol O₂, so the mole fraction of O₂ is 0.250/0.750 = 0.333. By Dalton's law, P(O₂) = 0.333 × 1.50 atm = 0.500 atm, even though O₂ is 80% of the mass.
7. A 40.0 g metal block at 98.0 °C is dropped into 60.0 g of water at 21.0 °C. The final temperature of both is 26.1 °C. Assuming no heat is lost to the cup or surroundings, what is the specific heat of the metal? (Water: 4.184 J/(g·°C).)
- A 0.198 J/(g·°C)
- B 0.297 J/(g·°C)
- C 0.416 J/(g·°C)
- D 0.445 J/(g·°C)
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Answer: D. 0.445 J/(g·°C)
Heat gained by the water = 60.0 g × 4.184 × (26.1 − 21.0) °C = 1280 J, and the metal loses the same amount. The metal cools by 98.0 − 26.1 = 71.9 °C, so c = 1280 J / (40.0 g × 71.9 °C) ≈ 0.445 J/(g·°C).
8. Using standard enthalpies of formation, CH₄(g) −74.8 kJ/mol, CO₂(g) −393.5 kJ/mol and H₂O(l) −285.8 kJ/mol, what is ΔH° for CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l)?
- A −890.3 kJ
- B −604.5 kJ
- C −1039.9 kJ
- D +890.3 kJ
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Answer: A. −890.3 kJ
ΔH° = ΣΔHf°(products) − ΣΔHf°(reactants) = [−393.5 + 2(−285.8)] − [−74.8 + 2(0)] = −890.3 kJ. O₂ is an element in its standard state, so its ΔHf° is zero, and the water term must be multiplied by its coefficient of 2.
9. What is the energy of one photon of light with a wavelength of 500. nm? (h = 6.626 × 10⁻³⁴ J·s, c = 3.00 × 10⁸ m/s.)
- A 3.98 × 10⁻²⁸ J
- B 6.00 × 10¹⁴ J
- C 3.98 × 10⁻¹⁹ J
- D 2.39 × 10⁵ J
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Answer: C. 3.98 × 10⁻¹⁹ J
E = hc/λ = (6.626 × 10⁻³⁴ J·s)(3.00 × 10⁸ m/s)/(500. × 10⁻⁹ m) = 3.98 × 10⁻¹⁹ J. Forgetting to convert nm to m gives 10⁻²⁸ J, and 2.39 × 10⁵ J is the energy of a whole mole of these photons.
10. What is the ground-state electron configuration of the Fe³⁺ ion? (Fe is element 26.)
- A [Ar]4s²3d³
- B [Ar]3d⁵
- C [Ar]4s¹3d⁴
- D [Ar]3d⁶
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Answer: B. [Ar]3d⁵
Neutral Fe is [Ar]4s²3d⁶. When a transition metal forms a cation, the 4s electrons are removed before the 3d electrons, so Fe³⁺ loses both 4s electrons and one 3d electron, leaving [Ar]3d⁵.
11. Arrange Na, Mg and Al in order of increasing first ionization energy.
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Answer: Na < Al < Mg
Ionization energy generally rises across a period, so Na is lowest. Al is below Mg because its outermost electron is in a 3p orbital, which is higher in energy and slightly shielded by the 3s electrons, so it is easier to remove than Mg's 3s electron.
12. Which of these isoelectronic ions has the largest radius?
- A Mg²⁺
- B Na⁺
- C F⁻
- D O²⁻
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Answer: D. O²⁻
All four ions have 10 electrons, so the one with the fewest protons holds them least tightly. O²⁻ has 8 protons, compared with 9 for F⁻, 11 for Na⁺ and 12 for Mg²⁺, so it is the largest.
13. According to VSEPR theory, what is the molecular geometry of SF₄?
- A Tetrahedral
- B Square planar
- C Seesaw
- D Trigonal pyramidal
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Answer: C. Seesaw
Sulfur has four bonding pairs and one lone pair, five electron domains in a trigonal bipyramidal arrangement. The lone pair sits in an equatorial position, which leaves the four fluorines in a seesaw shape.
14. In a Lewis structure of ozone, O₃, with one O=O double bond and one O–O single bond, what is the formal charge on the central oxygen atom?
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Answer: +1
Formal charge = valence electrons − nonbonding electrons − bonds. The central oxygen has one lone pair and three bonds, so its formal charge is 6 − 2 − 3 = +1 (the singly bonded end oxygen is −1).
15. Which of these compounds, all of similar molar mass, has the highest boiling point?
- A CH₃OCH₃
- B CH₃CH₂OH
- C CH₃CH₂CH₃
- D CH₃Cl
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Answer: B. CH₃CH₂OH
Ethanol has an O–H bond, so its molecules hydrogen-bond to one another, which is much stronger than the dipole-dipole forces in dimethyl ether and chloromethane or the dispersion forces in propane. Ethanol boils near 78 °C, while the other three boil below 0 °C.