Math practice · Unit #08 · 8th grade · Compounds, elements & mixtures

89% Oxygen

Water is H₂O — two hydrogens to one oxygen. So water is mostly hydrogen, right? Run the masses. The answer (89% oxygen) is why you always compute before you conclude.

skills: percent ratios two-way tables ≈ 35 min
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Name Period Date
PART A

Percent composition by mass

Atomic masses: H = 1, C = 12, O = 16. Mass percent of an element = (mass of that element in the formula ÷ total formula mass) × 100.

Fill in the masses, then the percents
SubstanceMass from each elementTotal mass% of biggest contributor
Water, H₂OH: 2×1 = 2 · O: 1618
Carbon dioxide, CO₂C: 12 · O: 2×16 = 3244
Glucose, C₆H₁₂O₆C: 6×12 = 72 · H: 12×1 = 12 · O: 6×16 = 96180

  1. Water: hydrogen outnumbers oxygen 2 to 1, yet oxygen owns ~89% of the mass. Resolve the contradiction in one sentence, using the word mass.

  2. Compute the carbon percent of CO₂ and of glucose. Which molecule is "carbonier" by mass? Does that match how they feel (a gas you exhale vs. a sugar you eat)?

PART B

There's metal in your breakfast

A fortified cereal label reads: iron, 8 mg per serving — 45% daily value. (And yes — a magnet can pull real iron filings out of the crushed cereal. An element, hiding in a mixture, that your body needs.)

  1. If 8 mg is 45% of the daily value, what is 100% — the full daily value? (Set it up: 8 ÷ 0.45)

  2. How many servings would it take to eat one full gram (1,000 mg) of iron? Careful with units.

  3. Is the iron in the cereal an element, a compound, or a mixture component? What does the magnet demo prove that the label only claims?

PART C

The two-way sort

Sort all twelve into the table. One item per cell-count — then answer below. Oxygen gas (O₂) · air · salt water · pure water · steel · pizza · gold · sugar (C₁₂H₂₂O₁₁) · milk · aluminum foil · muddy water · hydrogen gas (H₂)

Two-way classification
Element (one type of atom)Compound (bonded, fixed ratio)Mixture (not bonded)
Count
Your items

  1. What fraction of the twelve are mixtures? Write it as a fraction, a decimal, and a percent.

  2. Steel trips everyone: iron + carbon mixed, not bonded in a fixed ratio. Name one other item whose category fooled you at first and say why it belongs where you finally put it.

PART D

The air pie

Air is a mixture: about 78% nitrogen, 21% oxygen, 1% everything else (mostly argon).

  1. In 200 L of classroom air, how many liters are oxygen?

  2. How many liters are "everything else"?

  3. Your lungs pull oxygen from air, but air is 78% nitrogen you don't use. If you breathe in 500 L, how much of that volume was just along for the ride?

Answer key — teachers

Part A

Water: O = 16/18 ≈ 88.9% · CO₂: O = 32/44 ≈ 72.7% · Glucose: O = 96/180 ≈ 53.3%.

  1. Counting atoms ignores mass — one oxygen outweighs both hydrogens 16 to 2, so mass percent ≠ atom count.
  2. CO₂: 12/44 ≈ 27.3% C; glucose: 72/180 = 40% C. Glucose is carbonier by mass — fitting for an energy-rich food vs. a waste gas.

Part B

  1. 8 ÷ 0.45 ≈ 17.8 mg
  2. 1,000 ÷ 8 = 125 servings
  3. An element mixed into the food; the magnet physically extracts it — evidence beats label claims.

Part C

Elements (3): O₂, gold, aluminum, hydrogen… that's 4 — recount: O₂, Au, Al, H₂ = 4. Compounds (2): pure water, sugar. Mixtures (6): air, salt water, steel, pizza, milk, muddy water.

  1. 6/12 = 1/2 = 0.5 = 50% mixtures.
  2. Steel is a mixture (no fixed ratio); accept any well-argued second example.

Part D

  1. 0.21 × 200 = 42 L
  2. 0.01 × 200 = 2 L
  3. 0.78 × 500 = 390 L of nitrogen along for the ride.