Recipe Ratios
Find percentage composition of elements in compounds
What this lesson covers
Why it matters
A chocolate-chip cookie keeps the same chip-to-dough ratio whether you bake one or a hundred. A compound is the same: its elements are locked in a fixed ratio by mass.
Predict first
If you double the amount of water you have, what happens to the percentage of hydrogen by mass?
Percentage composition is fixed by the formula, not the amount. In water, two light H atoms (1 amu each) ride one heavy O atom (16 amu).
- Nothing — the ratio is fixed, so it stays about 11% — correct
- It doubles, because there is twice as much hydrogen
- It halves
What you do
Use the atomic masses (H=1, C=12, N=14, O=16) to work out what fraction of each compound's mass comes from the named element — then match each compound to its percentage composition.
Check yourself
What percentage of water's mass is hydrogen? (H=1, O=16)
Mass of H is 2, total is 18, so 2/18 ≈ 11%. Counting atoms (2 of 3) ignores how light hydrogen is.
Why does percentage composition stay the same for any amount of a pure compound?
A compound's fixed formula fixes its mass ratios — a defining difference from a mixture.
- About 11% (2 out of 18) — correct
- About 67% (2 atoms out of 3)
- 50%
- The elements are combined in a fixed ratio by mass — correct
- Compounds can have any ratio
- Only mixtures have fixed ratios