Read ppm, ppb and Percentage Composition as One System
ppm, ppb and percentage composition are all mass-of-X-per-mass-of-whole, scaled differently — get the scale factor right (10⁶ for ppm, 10⁹ for ppb) and the rest is arithmetic.
Chemistry questions first require the material, particles or reaction condition to be identified before a formula or rule is selected.
ppm and ppb are mass ratios, not mole ratios
Converting a ppb figure to molar concentration needs the substance's molar mass as an extra step — ppm/ppb alone never gives you molarity directly, whatever the question seems to imply.
Error propagation in a real lab scenario
When a student substitutes the wrong reagent quantity into a ppm calculation, the error propagates linearly through the whole result — isolate exactly which number was swapped before recomputing, rather than redoing the whole calculation from scratch.
Percentage composition from a molecular formula
Compute each element's total mass in one mole of compound from its subscript and atomic mass, divide by the compound's molar mass, and multiply by 100 — do this per element the question actually asks about.
OlympIQ exam lens
- ✓ppm = mass of solute / mass of solution × 10⁶; ppb uses 10⁹.
- ✓Converting a mass-based unit to molarity always needs molar mass as an extra step.
- ✓Percentage composition is computed per element from the molecular formula's own atom counts.
Don't treat ppm/ppb as automatically equivalent to molarity or mole fraction — they're mass ratios unless the question explicitly converts them.
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