Stoichiometric Mole Ratios UPCAT Reviewer: Lesson and Practice

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Stoichiometric Mole Ratios

Turn balanced coefficients into conversion factors without guessing proportions.

5-10 minute lesson27 original questionsAdaptive practiceSaves progress

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Stoichiometric Mole Ratios

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Balance, convert to moles, apply the ratio, then convert to the requested unit

Balanced coefficients represent particle and mole relationships—not gram relationships. A complete setup follows the units:

given unit → moles of given substance× coefficient ratio (wanted/given)→ requested unit

When two reactants are supplied, calculate how much product each could make. The smaller result identifies the limiting reactant.

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. Mole ratio

For N₂+3H₂→2NH₃:

6 mol H₂ × 2 mol NH₃ / 3 mol H₂ = 4 mol NH₃

2. Mass to mass

For 2H₂+O₂→2H₂O, 4 g H₂ is 2 mol H₂. The 2:2 ratio gives 2 mol H₂O, or 36 g.

3. Limiting reactant

For N₂+3H₂→2NH₃, 2 mol N₂ need 6 mol H₂. If only 3 mol H₂ are present, H₂ limits production to 2 mol NH₃.

4. Excess reactant

For 2H₂+O₂→2H₂O, 2 mol O₂ consume 4 mol H₂. Starting with 5 mol H₂ leaves 1 mol.

5. Percent yield

If theory predicts 25 g but the experiment obtains 20 g:

percent yield=20/25×100%=80%

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

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Core Practice

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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

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