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4.2.1Chemical Reactions

Conservation of Mass — AP CHEMISTRY Revision Notes & Topical Questions

5 min read 5 parts Lesson 14 of 18 Unit quiz

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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 5The Law

Mass is conserved in every chemical change, which allows unknown masses to be found by subtraction.

Statement
  • Statement In a chemical reaction in a closed system the total mass of the reactants equals the total mass of the products.
Atomic reason
  • Atomic reason Atoms are rearranged into new combinations but are not created or destroyed, so total mass stays the same.
Balanced equations
  • Balanced equations A balanced equation reflects this law by having equal numbers of each type of atom on both sides.
Closed system
  • Closed system If gas enters or escapes, the measured mass of the remaining solid or liquid appears to change.

Part 2 of 5Applying It

1
List the masses of all reactants and products that are known.
2
Write the mass balance: total mass of reactants = total mass of products.
3
Solve the equation for the unknown mass.
4
Check that your answer is positive and has sensible units.
5
Compare with the equation: the mass ratios should match the moles and molar masses from the balanced equation.

Part 3 of 5Apparent Mass Changes

Apparent Mass Changes
Metal burning in air
  • Metal burning in air A burned metal gains mass because it combines with oxygen from the air, and the oxide is heavier than the metal.
Carbonate decomposition
  • Carbonate decomposition Heating a carbonate loses mass because carbon dioxide gas escapes into the air.
Open container reactions
  • Open container reactions An acid reacting with a carbonate in an open beaker appears to lose mass as gas leaves.
Sealed container
  • Sealed container If the reaction occurs in a sealed vessel, the total mass is unchanged before and after.
ObservationExplanation
Mass increases on burning a metalOxygen from air joins the metal
Mass decreases on heating a carbonateCO₂ escapes as a gas

Part 4 of 5Worked Example

Question

  • 4.86 g of magnesium burns completely in air to form 8.06 g of magnesium oxide, MgO.
  • What mass of oxygen reacted?

  1. Write the mass balancemass of Mg + mass of O₂ = mass of MgO.
  2. 4.86 g + m(O₂) = 8.06 g.
  3. m(O₂) = 8.06 − 4.86 = 3.20 g.
  4. Check with moles4.86 g ÷ 24.31 g/mol = 0.200 mol Mg, which needs 0.100 mol O₂ = 3.20 g.

Answer 3.20 g of oxygen reacted.

Part 5 of 5Exam Checklist & Tips

4.2.1 – Key Points for the Exam

  • Law of conservation of mass Mass is neither created nor destroyed during a chemical reaction.
  • Closed system A system that exchanges no matter with its surroundings.
  • Reactant A starting substance in a reaction.
  • Product A substance formed in a reaction.

Common Mistakes

  • Forgetting that gases are part of the reaction and carry mass, which causes an apparent loss or gain.
  • Adding the masses of products on the reactant side, or the reverse, when setting up the equation.
  • Thinking mass is conserved but the number of atoms is not, which is the wrong way around.
  • Using the coefficients from the equation as masses instead of as ratios of moles.

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