Math practice · Unit #11 · 8th grade · Intro to chemical reactions

Nothing Is Lost

Atoms can't be created or destroyed — so chemistry is bookkeeping. Every atom that goes in comes out, and balancing an equation is just solving for the unknowns. It's algebra. The letters are atoms.

skills: solving for unknowns mass conservation balancing equations ≈ 40 min
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Name Period Date
PART A

Find x (it's mass)

mass of reactants = mass of products always, no appeals
Each reaction has one unweighed substance. The law of conservation of mass turns it into an equation. Solve for x.

  1. Wood 20 g + oxygen 15 g → ash 2 g + smoke & gases x. Find x.

  2. Iron 56 g + oxygen x → rust 80 g. Find x.

  3. Vinegar 100 g + baking soda x → everything after: 112 g (a gas escaped, but we caught it all). Find x.

  4. In problem 3, a classmate weighs the flask without catching the gas and finds it lighter than before. They declare "mass was destroyed!" What actually happened — and what does this say about open vs. closed systems?

PART B

The steel-wool ledger

The phenomenon: steel wool on a balance gains mass as it burns. Start: 28 g of steel wool (pure iron). After burning completely: 40 g of iron oxide.

  1. Mass of oxygen that joined the iron?

  2. Percent of the final product that is oxygen: (oxygen ÷ total) × 100 =

  3. Burning ADDED mass. Where was that mass hiding before the reaction — and why did nobody suspect it for centuries? (Hint: can you weigh air on a kitchen scale?)

PART C

Balancing act

Coefficients are the unknowns: find the smallest whole numbers that make every element's count match on both sides. Do the atom inventory first, then solve.

  1. __ H₂ + __ O₂ → __ H₂O (the classic)

  2. __ CH₄ + __ O₂ → __ CO₂ + __ H₂O (burning methane)

  3. __ Fe + __ O₂ → __ Fe₂O₃ (the steel wool, formalized)

  4. __ N₂ + __ H₂ → __ NH₃ (making fertilizer — the reaction that feeds half the world)

  5. Pick the one you found hardest. Write your solving steps as if explaining to someone absent today: which element did you balance first, and why?

PART D

Where does the smoke go? The full budget

Close the books on the campfire. Fill every shaded cell — each row and the grand total must balance.

Campfire mass budget (grams)
Before (reactants)After (products)
Wood500Ash left behind: 25
Oxygen (from air)420—
Carbon dioxide0
Water vapor0270
Total

  1. The CO₂ cell: total in − (ash + water vapor) = ?

  2. "Where does the smoke go?" is really "where does the WOOD go?" Answer in one sentence, citing two numbers from the table.

Answer key — teachers

Part A

  1. x = 20 + 15 − 2 = 33 g
  2. x = 80 − 56 = 24 g
  3. x = 112 − 100 = 12 g
  4. The gas escaped the flask — open systems leak mass; the law only balances when you count everything (closed system).

Part B

  1. 40 − 28 = 12 g
  2. 12 ÷ 40 = 30% (real Fe₂O₃ is 30.1% oxygen — the demo data holds up)
  3. The oxygen was invisible, weightless-seeming air; gases feel like nothing, so their mass fooled everyone until balances got precise.

Part C

  1. 2 H₂ + 1 O₂ → 2 H₂O
  2. 1 CH₄ + 2 O₂ → 1 CO₂ + 2 H₂O
  3. 4 Fe + 3 O₂ → 2 Fe₂O₃
  4. 1 N₂ + 3 H₂ → 2 NH₃
  5. Accept any clear strategy (balance metals first, save O/H for last, etc.).

Part D

Total before = 920 g · CO₂ = 920 − 25 − 270 = 625 g · Total after = 920 g ✓

  1. 625 g of CO₂.
  2. The wood mostly became gases — 625 g of it is now CO₂ in the air and 270 g is water vapor; only 25 g stayed as ash.