Tenth Grade · Science · week 23

Stoichiometry I: mole-to-mole conversions

Use the coefficients of a balanced chemical equation as mole ratios to convert from moles of one substance to moles of another.

Stoichiometry is the math of chemical reactions -- using a balanced equation to calculate exactly how much of one substance is needed or produced based on a given amount of another substance. It works because the coefficients in a balanced equation are not just a balancing trick; they tell you the exact mole RATIO in which substances react and form.

The mole ratio is built directly from the coefficients: in N2 + 3H2 -> 2NH3, the ratio of NH3 to N2 is 2 mol NH3 to 1 mol N2. To convert a known amount of one substance into another, multiply by that ratio as a conversion factor, with the substance you WANT on top and the substance you HAVE on the bottom, so the unwanted unit cancels.

📋 For the teacher
Teach it (10–15 min):
  • Insist the balanced equation is written and checked BEFORE any stoichiometry math begins -- an unbalanced equation gives a wrong mole ratio no matter how carefully the rest of the arithmetic is done.
  • Have them circle the two substances actually involved in a given problem directly on the equation and write their ratio as a small fraction off to the side before setting up the full conversion -- separating "find the ratio" from "do the conversion" reduces errors.
  • Reinforce the "what you want on top, what you have on the bottom" setup explicitly with unit cancellation, exactly like the dimensional analysis from early this year -- this is the same skill, applied to moles of different substances instead of different units.
  • Keep problems to mole-to-mole only this week; mass conversions are coming in the next two weeks and mixing them in now would blur which skill is actually new.
Talk about it:
  • Where does a mole ratio come from?
  • How do you decide which substance goes on top and which goes on the bottom of the mole ratio?
  • Why must an equation be balanced before you can trust its mole ratios?

Mastery looks like: They correctly set up and solve mole-to-mole stoichiometry problems using coefficients from a balanced equation as the conversion ratio.

Common stumbles: Using the coefficients from an unbalanced equation; flipping the mole ratio upside down so the wrong unit cancels.

Homework for this lessonType it, write it with a stylus, or print it for pencil & paper.open →
Records attendance + 15 min of Science for this learner.
← Back to todayAll coursesYear planner