Tenth Grade · Science · week 24

Stoichiometry II: mass-to-mass conversions

Convert from the mass of one substance to the mass of another using a balanced equation, molar mass, and mole ratios together in a single multi-step calculation.

Real laboratory measurements are made in grams, not moles, so the most practical stoichiometry problems start and end in mass. The path has three steps, each one a skill you already have: convert the given mass to moles using its molar mass, convert those moles to moles of the target substance using the mole ratio from the balanced equation, then convert those moles to a mass using the target substance's molar mass.

Written as a chain: grams (given) -> moles (given) -> moles (target) -> grams (target). Skipping the middle "moles" step and trying to relate two substances' masses directly is not possible, since the balanced equation only ever tells you a relationship in MOLES, never in grams directly.

📋 For the teacher
Teach it (10–15 min):
  • Write out the three-step chain (grams -> moles -> moles -> grams) as a literal template at the top of the page for the first several problems, and have them label which step they are on as they work -- the structure matters more than raw speed at this stage.
  • Have them look up and write down BOTH molar masses (the given substance and the target substance) before starting any arithmetic, so a mid-calculation pause to "go find the periodic table" does not break their concentration.
  • When an answer looks a little "off" from a round number, treat that as an opportunity to point out that real molar masses are rarely perfectly round, and rounding at the very end (not in the middle of the calculation) is what keeps the final answer as accurate as possible.
  • Connect this explicitly to last week: mole-to-mole was the middle step of this week's process the whole time, just with a grams-to-moles step bolted onto the front and a moles-to-grams step bolted onto the back.
Talk about it:
  • What are the three steps in a mass-to-mass stoichiometry problem, in order?
  • Why must every mass-to-mass problem pass through moles in the middle?
  • Why might a stoichiometry answer not come out to a perfectly round number?

Mastery looks like: They correctly complete a full mass-to-mass stoichiometry calculation, converting grams to moles, applying the correct mole ratio, and converting back to grams.

Common stumbles: Skipping the grams-to-moles conversion and trying to apply the mole ratio directly to grams; using the wrong substance's molar mass in the final conversion step.

Homework for this lessonType it, write it with a stylus, or print it for pencil & paper.open →
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