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General Chemistry I · Final exam review

General chemistry final exam review: practice test with answers

A Gen Chem 1 final pulls from the whole semester, but most of the math follows a handful of patterns. The fastest way to review is to find out which patterns you've forgotten, then practice those with units written out every time. This page gives you a formula sheet, three worked examples, and a 12-question practice test that tells you what to study next.

How to use this review

Rereading notes feels productive, but it doesn't show you what you can't do yet. A practice test does. So:

  1. Take the practice test cold. Use a calculator and the formula sheet below, nothing else. Give each question a real try before you check it.
  2. Look at your results. Below the test, the score box lists the topics you missed and where to practice them.
  3. Fix the top two or three topics. Then redo the questions you missed on a blank sheet, without opening the solutions.

The table shows what a typical Gen Chem 1 final covers. Your syllabus is the final word, since courses order and weight topics differently.

What a Gen Chem 1 final usually covers
TopicTypical questionWhere to practice
Measurement and sig figsRound a calculated answer; convert unitsWorked examples and practice test on this page
Moles and stoichiometryGrams of product from grams of reactantStoichiometry practice problems
Limiting reagent and yieldWhich runs out; theoretical and percent yieldLimiting reagent problems
Reactions in waterPrecipitates, net ionic equations, titrationsNet ionic equations practice
GasesPV = nRT, combined gas law, partial pressuresGas laws practice problems
Thermochemistryq = m·c·ΔT, calorimetry, ΔH from ΔH°fWorked examples and practice test on this page
Atoms and the periodic tableElectron configurations, ions, radius and ionization energy trendsWorked examples and practice test on this page
Bonding and shapesLewis structures, formal charge, VSEPR shapes, polarityLewis structures guide

Formula sheet

These eight relationships cover most of the calculations on a first-semester final. The third column is where points usually go missing.

Gen Chem 1 formula sheet
IdeaFormulaWatch out for
Moles from massn = mass ÷ molar massUse the molar mass of the whole formula, not one atom.
MolarityM = mol solute ÷ L solutionmL must become L. Divide by the solution volume, not the water added.
DilutionM1V1 = M2V2Both volumes in the same unit. The answer must be weaker than you started.
Ideal gas lawPV = nRT, R = 0.08206 L·atm/(mol·K)T in kelvin (°C + 273.15), P in atm, V in L.
Heatq = m·c·ΔT, c(water) = 4.184 J/(g·°C)ΔT = final − initial. Heat lost by one thing = heat gained by the other.
Enthalpy of reactionΔH° = ΣnΔH°f(products) − ΣnΔH°f(reactants)Multiply by coefficients. Elements in their standard state are 0.
Percent yieldactual ÷ theoretical × 100%Theoretical yield comes from the limiting reagent.
Empirical formula% → grams → moles → divide by the smallestAssume 100 g. Don't round 1.5 or 1.33 to a whole number; multiply up.

Useful molar masses for this page: H 1.008, C 12.01, O 16.00, Na 22.99, Mg 24.31, Cl 35.45 g/mol.

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Three worked examples, units carried through

Write a unit on every number in your setup. If the units don't cancel down to the unit you want, the setup is wrong, and you'll see it before you press a single calculator key.

Example 1: Percent yield

A student burns 2.40 g of magnesium and collects 3.60 g of MgO. What is the percent yield? 2 Mg + O2 → 2 MgO

  1. Moles of Mg: 2.40 g Mg × (1 mol Mg ÷ 24.31 g Mg) = 0.0987 mol Mg.
  2. Mole ratio: 0.0987 mol Mg × (2 mol MgO ÷ 2 mol Mg) = 0.0987 mol MgO.
  3. Theoretical yield: 0.0987 mol MgO × (40.31 g MgO ÷ 1 mol MgO) = 3.98 g MgO.
  4. Percent yield: (3.60 g ÷ 3.98 g) × 100% = 90.5%.

Unit check: g Mg cancels g Mg, mol Mg cancels mol Mg, mol MgO cancels mol MgO, leaving g MgO. In step 4, grams over grams leaves a pure percentage. A percent yield over 100% would mean a measuring mistake (or a wet product), not a great reaction.

Example 2: Calorimetry (heat lost = heat gained)

A 75.0 g piece of metal at 100.0 °C is dropped into 100.0 g of water at 20.0 °C. They end at 31.5 °C. Ignoring the cup, what is the specific heat of the metal?

  1. Heat gained by the water: q = (100.0 g)(4.184 J/(g·°C))(31.5 − 20.0 °C) = 4812 J. Keep the extra digit until the end.
  2. The metal loses the same amount, so qmetal = −4812 J.
  3. For the metal, ΔT = 31.5 − 100.0 = −68.5 °C.
  4. c = q ÷ (m·ΔT) = −4812 J ÷ (75.0 g × −68.5 °C) = 0.937 J/(g·°C).

Unit check: J ÷ (g·°C) is a specific heat. Sign check: the two negatives cancel, and a specific heat is always positive. If yours came out negative, one ΔT was written as initial − final.

Example 3: Empirical and molecular formula

A hydrocarbon is 85.6% carbon and 14.4% hydrogen by mass. Its molar mass is 56.1 g/mol. Find the empirical and molecular formulas.

  1. Assume 100 g, so you have 85.6 g C and 14.4 g H.
  2. Moles: 85.6 g ÷ 12.01 g/mol = 7.13 mol C. 14.4 g ÷ 1.008 g/mol = 14.3 mol H.
  3. Divide by the smaller number: C = 7.13 ÷ 7.13 = 1; H = 14.3 ÷ 7.13 = 2.00. Empirical formula: CH2 (14.03 g/mol).
  4. Molar mass ÷ empirical mass: 56.1 ÷ 14.03 = 4.00. Molecular formula: C4H8.

Check: 4(12.01) + 8(1.008) = 56.1 g/mol, which matches. If a ratio lands near 1.5 or 1.33, multiply every subscript by 2 or 3. Don't round it off.

Practice test: 12 questions with answers

For number questions, type the number without units, such as 43.9 or 4.00e-1. Answers within about 1% count, so rounding won't cost you. Question 7 needs a minus sign. If you're wrong, the feedback often names the exact slip, and you can try again. Every question has a full solution.

Part A: Moles and reactions

Question 1 Stoichiometry

How many grams of CO2 form when 16.0 g of CH4 burns completely? CH4 + 2 O2 → CO2 + 2 H2O

Show solution for question 1
  1. Grams to moles: 16.0 g CH4 × (1 mol ÷ 16.04 g) = 0.998 mol CH4.
  2. Mole ratio: 0.998 mol CH4 × (1 mol CO2 ÷ 1 mol CH4) = 0.998 mol CO2.
  3. Moles to grams: 0.998 mol × 44.01 g/mol = 43.9 g CO2.

Question 2 Limiting reagent

4.00 g of H2 reacts with 16.0 g of O2. How many grams of water can form? 2 H2 + O2 → 2 H2O

Show solution for question 2
  1. H2: 4.00 g ÷ 2.016 g/mol = 1.98 mol. O2: 16.0 g ÷ 32.00 g/mol = 0.500 mol.
  2. Using all the H2 would need 1.98 ÷ 2 = 0.992 mol O2. You only have 0.500 mol, so O2 is limiting.
  3. 0.500 mol O2 × (2 mol H2O ÷ 1 mol O2) = 1.00 mol H2O.
  4. 1.00 mol × 18.02 g/mol = 18.0 g H2O.

Part B: Solutions

Question 3 Molarity

5.85 g of NaCl is dissolved in enough water to make 250.0 mL of solution. What is the molarity?

Show solution for question 3
  1. 5.85 g NaCl ÷ 58.44 g/mol = 0.100 mol NaCl.
  2. 250.0 mL × (1 L ÷ 1000 mL) = 0.2500 L.
  3. M = 0.100 mol ÷ 0.2500 L = 0.400 M.

Question 4 Dilution

You dilute 25.0 mL of 6.00 M HCl to a final volume of 500.0 mL. What is the new concentration?

Show solution for question 4
  1. M1V1 = M2V2, so M2 = M1V1 ÷ V2.
  2. M2 = (6.00 M × 25.0 mL) ÷ 500.0 mL. The mL cancel, so you don't need to convert to liters here.
  3. M2 = 0.300 M.

Part C: Gases and energy

Question 5 Gas laws

What volume does 2.00 mol of an ideal gas occupy at 27 °C and 1.50 atm?

Show solution for question 5
  1. T = 27 + 273.15 = 300.15 K.
  2. V = nRT ÷ P = (2.00 mol)(0.08206 L·atm/(mol·K))(300.15 K) ÷ 1.50 atm.
  3. mol, K and atm cancel, leaving L: V = 32.8 L.

Question 6 Heat and calorimetry

How much heat, in kJ, does it take to warm 50.0 g of water from 20.0 °C to 45.0 °C? The specific heat of water is 4.184 J/(g·°C).

Show solution for question 6
  1. ΔT = 45.0 °C − 20.0 °C = 25.0 °C.
  2. q = m·c·ΔT = (50.0 g)(4.184 J/(g·°C))(25.0 °C) = 5230 J.
  3. 5230 J × (1 kJ ÷ 1000 J) = 5.23 kJ.

Question 7 Enthalpy of reaction

Use these standard enthalpies of formation to find ΔH° for CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(l): CH4(g) −74.6 kJ/mol, CO2(g) −393.5 kJ/mol, H2O(l) −285.8 kJ/mol. Include the sign.

Show solution for question 7
  1. Products: (−393.5) + 2(−285.8) = −965.1 kJ.
  2. Reactants: (−74.6) + 2(0) = −74.6 kJ. O2(g) is an element in its standard state, so its ΔH°f is 0.
  3. ΔH° = products − reactants = −965.1 − (−74.6) = −890.5 kJ. Negative means exothermic, as a combustion should be.

Part D: Atoms, trends and shapes

Question 8 Electron configurations

Iron is [Ar] 4s2 3d6. What is the electron configuration of Fe3+?

Show solution for question 8
  1. Fe has 26 electrons: [Ar] 4s2 3d6. Fe3+ has 23.
  2. Transition metals lose their 4s electrons before their 3d electrons. Remove 4s2, then one 3d electron.
  3. Fe3+ is [Ar] 3d5.

Question 9 Periodic trends

Which atom has the largest atomic radius?

Show solution for question 9
  1. Radius grows down a group: each row adds an occupied shell. K (row 4) is bigger than Na (row 3).
  2. Radius shrinks across a row: more protons pull the same shell in tighter. Mg is smaller than Na.
  3. Largest: K.

Question 10 Periodic trends

Which atom has the highest first ionization energy?

Show solution for question 10
  1. Na, Mg and Cl are all in row 3, so their outer electrons are in the same shell.
  2. Across a row, nuclear charge rises, so the outer electrons are held more tightly and ionization energy rises.
  3. Highest: Cl.

Question 11 Molecular shapes

What is the molecular shape of NH3, and about what is its H–N–H bond angle?

Show solution for question 11
  1. Lewis structure: N with three N–H bonds and one lone pair, so four electron groups.
  2. Four groups means tetrahedral electron geometry. With one lone pair, the shape of the atoms is trigonal pyramidal.
  3. The lone pair pushes harder than the bonds, so the angle is about 107°, a little under 109.5°.

Part E: Reactions in water

Question 12 Net ionic equations

Aqueous CaCl2 and Na3PO4 are mixed, and a precipitate forms. Which is the net ionic equation?

Show solution for question 12
  1. Swap partners: the possible products are Ca3(PO4)2 and NaCl. Most phosphates are insoluble, and NaCl is soluble, so the solid is Ca3(PO4)2.
  2. Na+ and Cl− appear unchanged on both sides, so they are spectators and drop out.
  3. Balance the charge: 3 Ca2+ (6+) and 2 PO43− (6−). Net ionic: 3 Ca2+(aq) + 2 PO43−(aq) → Ca3(PO4)2(s).

Answer key

  1. 43.9 g CO2
  2. 18.0 g H2O (O2 is the limiting reagent)
  3. 0.400 M
  4. 0.300 M
  5. 32.8 L
  6. 5.23 kJ
  7. −890.5 kJ
  8. [Ar] 3d5
  9. K
  10. Cl
  11. Trigonal pyramidal, about 107°
  12. 3 Ca2+(aq) + 2 PO43−(aq) → Ca3(PO4)2(s)

Points you lose for free

These aren't hard chemistry. They're the small slips that cost points on problems you actually know how to do.

  • Significant figures. When you multiply or divide, keep the fewest sig figs of any measured number: 3.20 × 1.5 = 4.8, not 4.80. When you add or subtract, keep the fewest decimal places: 31.5 − 20.0 = 11.5. Round once, at the end, not at every step.
  • Units. Write units on every number in the setup. mL into a molarity, J when the question asks for kJ, and g when the question wants mol are all caught by a unit check.
  • Kelvin. Any gas law problem with °C plugged in is wrong. Add 273.15 first. (ΔT in q = m·c·ΔT is the exception: a change of 1 °C is the same size as a change of 1 K.)
  • Coefficients. In ΔH°f and stoichiometry problems, forgetting to multiply by the 2 in 2 H2O is the classic mistake. So is using the subscript as a mole ratio.
  • Signs. Exothermic means ΔH is negative. In calorimetry, the heat lost by the hot object is the negative of the heat gained by the cold one.
  • Answering the question asked. Many wrong answers on this page are correct in-between numbers: moles when the question wanted grams, or J when it wanted kJ. Reread the question before you box your answer.

Frequently asked questions

What is on a general chemistry 1 final?

Usually measurement and significant figures, moles and stoichiometry, limiting reagents, solutions, gas laws, thermochemistry, atomic structure and electron configurations, periodic trends, and bonding and molecular shapes. Courses differ, so your syllabus and old exams are the final word.

How should I study for a gen chem final in a week?

Take a practice test first, before you review anything. Sort your misses by topic and spend most of your time on the two or three topics that cost you the most. Then redo every missed problem from a blank page without looking at the solution.

Do I need to memorize formulas for gen chem?

Many courses give you a formula sheet or a table of constants, but you still need to know when each formula applies and what units it needs. Ask your instructor what will be provided, and practice with only that.

Is the ACS general chemistry exam like this?

Some schools use a standardized ACS exam as the final. It is multiple choice, but it covers the same core topics, and many of its questions are the same calculations in disguise. Ask your instructor which exam your course uses.

Sources

Found the topics that need work? Practice them chapter by chapter, with feedback on every step, before the final.

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