Stoichiometric Mole Ratios UPCAT Reviewer: Lesson and Practice

TEACHER ABI UPCAT SCIENCE

Stoichiometric Mole Ratios

Turn balanced coefficients into conversion factors without guessing proportions.

5-10 minute lesson27 original questionsAdaptive practiceSaves progress

LessonShortcutExamplesSkill CheckPracticeMasteryFAQ

MY STATUS

Stoichiometric Mole Ratios

Not yet verified on this browser.

NOT STARTED

QUICK REVIEW

Balanced coefficients are mole relationships

A balanced chemical equation describes particle and mole ratios—not gram ratios.

N₂ + 3H₂ → 2NH₃

This means 1 mol N₂ reacts with 3 mol H₂ to form 2 mol NH₃.

Balance first

Never take a mole ratio from an unbalanced equation.

Units guide the setup

Arrange the conversion factor so the starting unit cancels.

DO IT FAST

Given × wanted/given

Write the given moles, then multiply by a coefficient fraction with the wanted substance on top and the given substance below.

Why it works

The balanced coefficients describe proportional amounts, so the conversion factor equals one reaction-scale relationship.

WORKED EXAMPLES

Five forms you should recognize

1. Read a ratio

For N₂ + 3H₂ → 2NH₃, N₂:NH₃ = 1:2.

2. Moles to moles

4 mol H₂ × (2 mol NH₃ / 3 mol H₂) = 8/3 mol NH₃.

3. Reverse direction

6 mol NH₃ × (3 mol H₂ / 2 mol NH₃) = 9 mol H₂.

4. Coefficient is not a subscript

The 2 before NH₃ counts two formula units or moles; the subscript 3 belongs inside each NH₃ molecule.

5. Mass problems

Convert grams to moles first, apply the mole ratio, then convert moles to grams.

COMMON TRAPS

Check before you commit

• Using subscripts as mole ratios
• Using an unbalanced equation
• Putting the conversion factor upside down
• Treating coefficients as gram ratios
• Skipping the grams-to-moles step
• Rounding too early

FIVE-FORM SKILL CHECK

Do you need the lesson-or just practice?

One original question in each form recommends your next step. It does not yet verify mastery.

CHOOSE YOUR PRACTICE

Work at the level you need.

Foundations

Build the core procedure with immediate explanations.

Core Practice

Use mixed forms with less scaffolding.

UPCAT-Style Transfer

Apply the competency in unfamiliar representations.

FRESH MASTERY CHECK

Ready to verify this competency?

A score of 5/5 verifies mastery. An unsuccessful attempt loads a different five-form bank.

QUICK ANSWERS

Stoichiometric Mole Ratios FAQ

Do coefficients give volume ratios?

For gases at the same temperature and pressure, coefficients can also represent volume ratios.

When do I use molar mass?

Use it when converting between grams and moles, not for a pure mole-to-mole conversion.

RELATED COMPETENCIES

Continue your science review.

Balancing EquationsMole ConceptLimiting Reactants

SAVE AND CONTINUE

Your progress stays on this browser.

Mastery results save to your Teacher Abi study profile.

Return to Student Hub View UPCAT Coverage