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Stoichiometry: Calculate Masses from Balanced Equations

Quantitative chemistry • Amount of substance and mass

Key concepts

What you'll likely be quizzed about

  • The mole provides a connection between particle count and mass.
  • One mole equals 6.02 × 10^23 particles.
  • Relative atomic masses (Ar) and relative formula masses (Mr) specify the mass of one mole of atoms or formula units in grams.
  • Consequently, the relationship moles = mass ÷ Mr illustrates that a change in mass causes a proportional change in the amount of moles.

Flashcards

Test your knowledge with interactive flashcards

Given 2Fe + O2 → 2FeO, find mass of FeO formed from 11.2 g Fe (Ar Fe = 55.8, O = 16.0).

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Moles Fe = 11.2 ÷ 55.8 = 0.201 mol. Ratio Fe:FeO = 1:1 leads to moles FeO = 0.201 mol. Mr FeO = 55.8 + 16.0 = 71.8. Mass = 0.201 × 71.8 ≈ 14.4 g.

Key notes

Important points to keep in mind

Convert mass to moles before using mole ratios.

Use coefficients from the balanced equation directly as mole ratios.

Determine the limiting reagent by comparing available moles to required moles.

For ratios yield fractions like 1.5, multiply by 2 for whole numbers.

Maintain significant figures; round only at the final step.

Verify units at each step: g ↔ mol ↔ g.

Ensure conservation of mass is validated when applicable.

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