In 2H2 + O2 → 2H2O, how many moles of water form from 3 moles of O2?
Answer: 6 moles of H2O.
Explanation
Ratio of O2 to H2O is 1:2.
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Chemistry: Stoichiometry and Chemical Reactions (High School)
StoichiometryRedoxAcids & basesSolutionsEnergyGasesMolesReaction typesEquations
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Stoichiometry
What is the excess reactant?
Answer: The reactant left over after the limiting reactant is consumed.
Explanation
Leftover = initial − amount used.
How do you calculate percent yield?
Answer: (actual yield / theoretical yield) × 100
Explanation
Theoretical yield comes from stoichiometry with the limiting reactant.
What is the limiting reactant?
Answer: The reactant that runs out first and determines the maximum product.
Explanation
Compute product from each reactant; the smaller amount wins.
What is the general stoichiometry roadmap?
Answer: Grams A → moles A (molar mass) → moles B (mole ratio from coefficients) → grams B (molar mass).
Explanation
Always start from a balanced equation.
Redox
How do you identify oxidation and reduction?
Answer: Oxidation is loss of electrons (oxidation number rises); reduction is gain of electrons (OIL RIG).
Explanation
The oxidizing agent is itself reduced.
Acids & bases
Arrhenius vs Brønsted-Lowry definitions?
Answer: Arrhenius: acids produce H⁺ and bases produce OH⁻ in water. Brønsted-Lowry: acids donate protons, bases accept protons.
Explanation
Brønsted-Lowry covers ammonia as a base.
What is the pH scale?
Answer: A measure of hydrogen ion concentration: pH = −log[H⁺]; below 7 acidic, 7 neutral, above 7 basic.
Explanation
Each unit is a tenfold change.
Solutions
How does temperature affect solubility of solids and gases?
Answer: Most solids dissolve more at higher temperature; gases dissolve less.
Explanation
Warm soda goes flat faster.
What is a saturated solution?
Answer: A solution holding the maximum amount of dissolved solute at a given temperature.
Explanation
Supersaturated solutions hold more than that, unstably.
Energy
What is specific heat and the heat equation?
Answer: The heat needed to raise 1 g of a substance by 1 °C; q = mcΔT.
Explanation
Water's specific heat is 4.18 J/g·°C.
What is activation energy and how do catalysts affect it?
Answer: The minimum energy needed to start a reaction; catalysts lower it without being consumed.
Explanation
Enzymes are biological catalysts.
Exothermic vs endothermic?
Answer: Exothermic releases heat (ΔH negative); endothermic absorbs heat (ΔH positive).
Explanation
Combustion is exothermic; melting ice is endothermic.
Gases
What is Dalton's law of partial pressures?
Answer: Total pressure equals the sum of the partial pressures of each gas.
Explanation
Partial pressure = mole fraction × total pressure.
What are Boyle's, Charles's, and Gay-Lussac's laws?
Answer: Boyle: P1V1 = P2V2 (constant T). Charles: V1/T1 = V2/T2 (constant P). Gay-Lussac: P1/T1 = P2/T2 (constant V).
Explanation
Combined gas law: P1V1/T1 = P2V2/T2.
State the ideal gas law.
Answer: PV = nRT, with R = 0.0821 L·atm/(mol·K) and T in kelvin.
Explanation
K = °C + 273.
Moles
How do you convert grams to moles?
Answer: Divide the mass by the molar mass (g/mol) from the periodic table.
Explanation
Moles to grams: multiply by molar mass.
Empirical vs molecular formula?
Answer: Empirical is the simplest whole-number ratio of atoms; molecular is the actual number of atoms.
Explanation
Glucose: empirical CH2O, molecular C6H12O6.
What is percent composition?
Answer: The mass percentage of each element in a compound: (mass of element / molar mass) × 100.
Explanation
Water is about 11% hydrogen, 89% oxygen by mass.
What is molarity?
Answer: Moles of solute per liter of solution: M = mol / L.
Explanation
Dilution: M1V1 = M2V2.
What volume does one mole of gas occupy at STP?
Answer: 22.4 liters.
Explanation
STP = 0 °C and 1 atm.
How many particles are in one mole?
Answer: 6.022 × 10²³ (Avogadro's number).
Explanation
Particles → moles: divide by Avogadro's number.
Reaction types
What is a neutralization reaction?
Answer: An acid and a base react to form water and a salt: HCl + NaOH → NaCl + H2O.
Explanation
Net ionic: H⁺ + OH⁻ → H2O.
What drives a double replacement reaction to occur?
Answer: Formation of a precipitate, a gas, or water (in neutralization).
Explanation
Use solubility rules to predict precipitates.
What happens in a single replacement reaction?
Answer: A more reactive element displaces a less reactive one in a compound: Zn + CuSO4 → ZnSO4 + Cu.
Explanation
Use the activity series to predict.
What is a combustion reaction?
Answer: A hydrocarbon reacts with oxygen to produce carbon dioxide and water, releasing energy.
Explanation
Incomplete combustion produces carbon monoxide.
What are the five main reaction types?
Answer: Synthesis (A + B → AB), decomposition (AB → A + B), single replacement, double replacement, combustion.
Explanation
Acid-base and redox are additional classifications.
Equations
Balance: __C3H8 + __O2 → __CO2 + __H2O
Answer: C3H8 + 5O2 → 3CO2 + 4H2O
Explanation
Balance C, then H, then O last for combustion.
Balance: __H2 + __O2 → __H2O
Answer: 2H2 + O2 → 2H2O
Explanation
Balance oxygen first, then hydrogen.
Why must chemical equations be balanced?
Answer: To obey the law of conservation of mass: atoms are neither created nor destroyed.
Explanation
Adjust coefficients, never subscripts.
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