Physical Science: Chemistry — Midterm exam worked answers
Student exam paperAn independent assessment of the core chapters listed below, followed by corrections and discussion.
18 questions · 42 points · suggested 60 minutes
Student instructions
- Suggested time: 60 minutes; allow breaks or accommodations and record them.
- Work independently with a calculator, blank paper and the reference relationships printed in this paper. Grades 9, 11 and 12 may also use the course-linked NYSED reference tables. Use no lesson notes or answer key during an assessed attempt.
- Show reasoning and units for written tasks. All supplied datasets are illustrative; do not present them as measurements you collected.
- Objective items: 1 point each. Written items: 3 points each, one per criterion. A parent or teacher marks the written work; the app never grades an explanation automatically.
- Sample exam: try the whole paper before opening the worked key, then revisit the linked chapters. The sample and final are different papers; this is a course assessment, not an official Regents examination.
Reference relationships
| Relationship, constant or reference | Use / conditions |
|---|---|
| Density | ρ=m/V; Mass and volume in consistent units |
| Amount | n=m/Mm; N=nNA; Mm is molar mass, not molarity |
| Concentration | M=n/V; M₁V₁=M₂V₂; Solution liters; dilution without reaction or loss |
| Ideal gas | PV=nRT; Kelvin and absolute pressure; R matched to units |
| Heating | q=mcΔT; qphase=mL; One phase versus phase change |
| Reaction heat | qreaction≈−qsurroundings; Include apparatus/loss terms when needed |
| Free energy | ΔG=ΔH−TΔS; Kelvin and matching energy units; not reaction rate |
| Equilibrium | Q, K=products/reactants with powers; Omit pure phases in the school approximation |
| Acidity | pH≈−log[H₃O⁺]; Concentration approximation; neutral near 7 at 25°C |
| Buffer | pH≈pKa+log([A⁻]/[HA]); Both conjugate species appreciable |
| Cell potential | E°cell=E°cathode−E°anode; Both table values are reduction potentials |
| Radioactive parent | N=N₀(½)^(t/thalf); Closed parent accounting; statistical model |
| NA=6.022×10²³ mol⁻¹ | Avogadro constant, rounded for school calculations |
| h=6.626×10⁻³⁴ J·s | Planck constant, rounded |
| c=3.00×10⁸ m/s | Vacuum light speed, rounded |
| e=1.602×10⁻¹⁹ C | Elementary charge magnitude; 1 eV=1.602×10⁻¹⁹ J |
| g=9.8 m/s² | Typical near-surface Earth approximation; use a supplied local value if given |
| R=8.314 J/(mol·K) | Use SI pressure Pa and volume m³ |
| R=0.0821 L·atm/(mol·K) | Use atmosphere pressure and liter volume |
| F≈96485 C/mol electrons | Faraday constant |
| 1 u·c²≈931.5 MeV | Mass-energy conversion for consistent nuclear mass accounting |
| T(K)=T(°C)+273.15 | Use 273 only when a question specifies that rounded convention |
| 1 L=1000 mL; 1 mL=1 cm³ | Volume conversions |
| 1 kWh=3.60×10⁶ J | Energy conversion |
| cwater≈4.18 J/(g·°C) | Approximate liquid-water specific heat in ordinary conditions |
| Lf,water≈334 J/g; Lv,water≈2260 J/g | Water fusion and vaporization near ordinary-pressure transition conditions |
| H | 1.008 g/mol |
| C | 12.01 g/mol |
| N | 14.01 g/mol |
| O | 16.00 g/mol |
| Na | 22.99 g/mol |
| Mg | 24.31 g/mol |
| Al | 26.98 g/mol |
| P | 30.97 g/mol |
| S | 32.06 g/mol |
| Cl | 35.45 g/mol |
| K | 39.10 g/mol |
| Ca | 40.08 g/mol |
| Fe | 55.85 g/mol |
| Cu | 63.55 g/mol |
| Zn | 65.38 g/mol |
| Ag | 107.87 g/mol |
| NH₄⁺ / OH⁻ / NO₃⁻ | Ammonium +1; hydroxide −1; nitrate −1 |
| CO₃²⁻ / SO₄²⁻ / PO₄³⁻ | Carbonate −2; sulfate −2; phosphate −3 |
Coverage and marks
| Chapter | Questions | Points |
|---|---|---|
| Investigating Matter | 1, 2, 3 | 7 |
| Atoms, Electrons & Periodicity | 4, 5, 6 | 7 |
| Bonding, Formulas & Molecular Structure | 7, 8, 9 | 7 |
| Reactions, Moles & Composition | 10, 11, 12 | 7 |
| Stoichiometry & Yield | 13, 14, 15 | 7 |
| States of Matter & Gases | 16, 17, 18 | 7 |
Parent / teacher marking guide
- Mark objective answers after submission. Written marks remain pending until a parent or teacher applies the three-point criteria.
- Accept scientifically equivalent wording and valid alternative methods. Award stated method credit for a correct model or substitution even if later arithmetic fails.
- A raw percentage describes this paper only; it is not a validated course grade or a certification. Review chapter-level evidence and the learner’s independent explanations.
- Record support and accommodations, save corrections and reassess with different tasks. Do not count copied solutions as independent mastery.
Question 1 · 1 point
A mixture contains 5 g salt and 8 g sand. After complete physical separation, what total dry solid mass should be recovered in a closed no-loss model?
Worked answer
13 g
5+8=13; material is redistributed, not created.
1 point for the correct objective answer; no written-method marks for this item.
Question 2 · 3 points
A sample has mass 24.0 g and volume 8.00 mL. Find density.
Worked answer
3.00 g/mL
Write ρ = m/V.
Substitute 24.0 g / 8.00 mL.
Three significant figures are appropriate.
Marking criteria
- 1 point: identify the appropriate model or mechanism for this task.
- 1 point: show the calculation or evidence in the worked steps: Write ρ = m/V.
- 1 point: reach a scientifically supported conclusion: 3.00 g/mL
Question 3 · 3 points
Evaluate the evidence in this supplied classroom investigation: Density and Measurement Evidence. State a supported claim, cite two specific entries from the dataset, and identify one limitation plus an improvement.
Worked answer
Mean masses are 10,20,30 g; all give 1.00 g/mL. Repeats span.2 g, supporting approximate consistency in this illustrative range. A known calibration mass or independent volume standard would address accuracy; the repeats alone do not exclude common bias.
Read the column labels and compare entries using their stated units.
Connect the observed pattern to the investigation mechanism, not just a repeated description.
Evaluate the supplied example data; do not claim they establish every real-world case.
Marking criteria
- 1 point: a claim consistent with the supplied dataset and scientific mechanism.
- 1 point: cite and correctly compare at least two specific data entries.
- 1 point: a relevant limitation and improvement addressing it.
Question 4 · 1 point
An atom of selenium has atomic number 34 and mass number 80. How many neutrons does it have?
Worked answer
46
Neutrons = A − Z = 80 − 34 = 46.
1 point for the correct objective answer; no written-method marks for this item.
Question 5 · 3 points
An atom is written in isotope notation with mass number 27 and atomic number 13 (aluminum). Find its numbers of protons, neutrons, and electrons.
Worked answer
13 protons, 14 neutrons, 13 electrons
Protons = atomic number Z = 13.
Neutrons = mass number A − atomic number Z = 27 − 13 = 14.
Electrons = protons in a neutral atom = 13.
Marking criteria
- 1 point: identify the appropriate model or mechanism for this task.
- 1 point: show the calculation or evidence in the worked steps: Protons = atomic number Z = 13.
- 1 point: reach a scientifically supported conclusion: 13 protons, 14 neutrons, 13 electrons
Question 6 · 3 points
Evaluate the evidence in this supplied classroom investigation: Spectrum Evidence and Identification. State a supported claim, cite two specific entries from the dataset, and identify one limitation plus an improvement.
Worked answer
Unknown matchesA within 2 nm at every supplied line.450 nm has highest photon energy and 650 nm lowest. Identity is supported within this simplified reference set, not proven among all possible elements or mixtures.
Read the column labels and compare entries using their stated units.
Connect the observed pattern to the investigation mechanism, not just a repeated description.
Evaluate the supplied example data; do not claim they establish every real-world case.
Marking criteria
- 1 point: a claim consistent with the supplied dataset and scientific mechanism.
- 1 point: cite and correctly compare at least two specific data entries.
- 1 point: a relevant limitation and improvement addressing it.
Question 7 · 1 point
Aluminum forms Al³⁺ and oxygen forms O²⁻. In the neutral compound aluminum oxide, how many oxide ions combine with every 2 aluminum ions?
Worked answer
3
Two Al³⁺ give +6, so you need three O²⁻ (−2 each) to reach −6 and balance the charge: Al₂O₃.
1 point for the correct objective answer; no written-method marks for this item.
Question 8 · 3 points
Predict the formula of the ionic compound formed between magnesium and chlorine.
Worked answer
MgCl₂
Find each ion's charge from its group. Magnesium is in Group 2, so it loses 2 electrons to form Mg²⁺. Chlorine is in Group 17, so it gains 1 electron to form Cl⁻.
Balance the charges so the compound is neutral: one Mg²⁺ (+2) needs two Cl⁻ (−1 each) to cancel it.
Write the formula with those subscripts: one Mg and two Cl.
Marking criteria
- 1 point: identify the appropriate model or mechanism for this task.
- 1 point: show the calculation or evidence in the worked steps: Find each ion's charge from its group. Magnesium is in Group 2, so it loses 2 electrons to form Mg²⁺. Chlorine is in Group 17, so it gains 1 electron to form Cl⁻.
- 1 point: reach a scientifically supported conclusion: MgCl₂
Question 9 · 3 points
Evaluate the evidence in this supplied classroom investigation: Bond Models and Material Properties. State a supported claim, cite two specific entries from the dataset, and identify one limitation plus an improvement.
Worked answer
CO₂ bond dipoles cancel in its linear geometry. Salt conduction requires mobile ions, while copper has mobile electrons in the solid. Property predictions require structure and phase; a single property is not a unique identity test.
Read the column labels and compare entries using their stated units.
Connect the observed pattern to the investigation mechanism, not just a repeated description.
Evaluate the supplied example data; do not claim they establish every real-world case.
Marking criteria
- 1 point: a claim consistent with the supplied dataset and scientific mechanism.
- 1 point: cite and correctly compare at least two specific data entries.
- 1 point: a relevant limitation and improvement addressing it.
Question 10 · 1 point
Classify the reaction 2KClO₃ → 2KCl + 3O₂.
- synthesis
- decomposition
- single replacement
- double replacement
Worked answer
decomposition
A single compound breaking apart into simpler substances is decomposition (AB → A + B).
1 point for the correct objective answer; no written-method marks for this item.
Question 11 · 3 points
Classify this reaction and explain your choice: 2Na + Cl₂ → 2NaCl.
Worked answer
Synthesis reaction — two elements combine to form one compound.
Look at the pattern: two separate elements (sodium and chlorine) appear on the left, and a single compound (sodium chloride) appears on the right.
Two or more substances combining into one product matches the form A + B → AB.
That is the definition of a synthesis (combination) reaction.
Marking criteria
- 1 point: identify the appropriate model or mechanism for this task.
- 1 point: show the calculation or evidence in the worked steps: Look at the pattern: two separate elements (sodium and chlorine) appear on the left, and a single compound (sodium chloride) appears on the right.
- 1 point: reach a scientifically supported conclusion: Synthesis reaction — two elements combine to form one compound.
Question 12 · 3 points
Evaluate the evidence in this supplied classroom investigation: Atom Accounting and Ionic Change. State a supported claim, cite two specific entries from the dataset, and identify one limitation plus an improvement.
Worked answer
Coefficients count unchanged chemical formulas; modifying subscripts changes identity. Na⁺ andNO₃⁻ are spectators in the stated reaction. Mass-to-mole conversion accounts for different elemental molar masses before deriving atom ratios.
Read the column labels and compare entries using their stated units.
Connect the observed pattern to the investigation mechanism, not just a repeated description.
Evaluate the supplied example data; do not claim they establish every real-world case.
Marking criteria
- 1 point: a claim consistent with the supplied dataset and scientific mechanism.
- 1 point: cite and correctly compare at least two specific data entries.
- 1 point: a relevant limitation and improvement addressing it.
Question 13 · 1 point
For N₂ + 3H₂ → 2NH₃, how many moles of ammonia form from 9.0 mol of hydrogen (with excess nitrogen)?
Worked answer
6 mol
9.0 mol H₂ × (2 mol NH₃ ÷ 3 mol H₂) = 6.0 mol NH₃.
1 point for the correct objective answer; no written-method marks for this item.
Question 14 · 3 points
For the reaction N₂ + 3H₂ → 2NH₃, how many moles of ammonia (NH₃) can be produced from 6.0 mol of hydrogen (H₂) reacting with excess nitrogen?
Worked answer
4.0 mol of NH₃
Read the mole ratio from the coefficients: NH₃ to H₂ is 2 : 3.
Multiply the given moles of H₂ by the ratio, arranged so H₂ cancels: 6.0 mol H₂ × (2 mol NH₃ ÷ 3 mol H₂).
Compute: 6.0 × 2 ÷ 3 = 4.0 mol NH₃.
Marking criteria
- 1 point: identify the appropriate model or mechanism for this task.
- 1 point: show the calculation or evidence in the worked steps: Read the mole ratio from the coefficients: NH₃ to H₂ is 2 : 3.
- 1 point: reach a scientifically supported conclusion: 4.0 mol of NH₃
Question 15 · 3 points
Evaluate the evidence in this supplied classroom investigation: Limiting-Reactant Production Model. State a supported claim, cite two specific entries from the dataset, and identify one limitation plus an improvement.
Worked answer
Theoretical yields are 2,4,4 mol. H₂ limits run 1; both are stoichiometric inrun 2; N₂ limits run 3. Yields 90%,90%,87.5%. Actual shortfall could reflect losses, incomplete conversion, or side reactions; these numbers alone do not identify a cause.
Read the column labels and compare entries using their stated units.
Connect the observed pattern to the investigation mechanism, not just a repeated description.
Evaluate the supplied example data; do not claim they establish every real-world case.
Marking criteria
- 1 point: a claim consistent with the supplied dataset and scientific mechanism.
- 1 point: cite and correctly compare at least two specific data entries.
- 1 point: a relevant limitation and improvement addressing it.
Question 16 · 1 point
Convert 27°C to kelvin.
Worked answer
300 K
K = °C + 273 = 27 + 273 = 300 K.
1 point for the correct objective answer; no written-method marks for this item.
Question 17 · 3 points
A heating curve for a pure substance shows temperature rising, then holding flat at 80°C, then rising again, then holding flat at 218°C, then rising once more. Identify what is happening at each of the two flat plateaus.
Worked answer
The 80°C plateau is melting (solid to liquid); the 218°C plateau is boiling (liquid to gas).
A flat plateau on a heating curve means energy is still flowing in but the temperature is not changing — the signature of a phase change.
The lower plateau (80°C) is the first change as the substance warms from solid: it is melting (solid to liquid) at its melting point.
The higher plateau (218°C) is the next change up the curve: it is boiling (liquid to gas) at its boiling point.
Marking criteria
- 1 point: identify the appropriate model or mechanism for this task.
- 1 point: show the calculation or evidence in the worked steps: A flat plateau on a heating curve means energy is still flowing in but the temperature is not changing — the signature of a phase change.
- 1 point: reach a scientifically supported conclusion: The 80°C plateau is melting (solid to liquid); the 218°C plateau is boiling (liquid to gas).
Question 18 · 3 points
Evaluate the evidence in this supplied classroom investigation: Heating-Curve Energy Budget. State a supported claim, cite two specific entries from the dataset, and identify one limitation plus an improvement.
Worked answer
Melting takes 3340 J; warming liquid takes 836 J; total 4176 J. The plateau represents phase rearrangement energy. Real equipment loses heat and may have sensor gradients, so measured input can exceed the ideal amount.
Read the column labels and compare entries using their stated units.
Connect the observed pattern to the investigation mechanism, not just a repeated description.
Evaluate the supplied example data; do not claim they establish every real-world case.
Marking criteria
- 1 point: a claim consistent with the supplied dataset and scientific mechanism.
- 1 point: cite and correctly compare at least two specific data entries.
- 1 point: a relevant limitation and improvement addressing it.