Stoichiometry: calculate masses from balanced equations

Quantitative chemistry • Amount of substance and mass

Flashcards

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How do coefficients in an equation affect the amounts of products formed?

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Coefficients set mole ratios; increasing moles of a reactant increases possible product moles proportionally until a limiting reagent is reached.

Key concepts

What you'll likely be quizzed about

Definition of mole and molar mass

The mole provides a bridge between particle count and mass. One mole equals 6.02 × 10^23 particles. Relative atomic masses (Ar) and relative formula masses (Mr) describe the mass of one mole of atoms or formula units in grams. Mass and moles relate by the equation moles = mass ÷ Mr, so a change in mass causes a proportional change in mole amount.

Mole ratios from balanced equations

Coefficients in a balanced symbol equation indicate the ratio of moles of reactants and products. Using the mole ratio converts moles of a known substance into moles of an unknown substance. For example, in 2A + B → 3C, every 2 moles of A react with 1 mole of B to form 3 moles of C; therefore increasing the available moles of A increases the possible moles of C proportionally, until another reagent limits the reaction.

Stepwise mass calculation method

Mass calculations proceed in three steps: convert known mass to moles (moles = mass ÷ Mr), use the balanced equation to find moles of the target substance via mole ratio, then convert those moles to mass (mass = moles × Mr). This chain (mass → moles → mole ratio → moles → mass) prevents unit mismatch and clarifies the causal link between a given mass and the required or produced mass.

Limiting reagent and excess reagent

The limiting reagent is the reactant that runs out first and therefore determines the maximum mass of product formed. Comparison of available moles against required mole ratios identifies the limiting reagent: convert each given mass to moles, compare the mole ratio to the balanced equation, and identify which reagent is insufficient. Excess reagents remain after the limiting reagent is consumed and do not increase product mass.

Balancing equations from masses

Balancing an equation from given masses uses mass → moles conversions to find relative mole amounts, then simplifies those mole amounts to whole-number coefficients. Convert each substance mass to moles, divide all moles by the smallest mole value to obtain ratios, then multiply by an integer to clear fractions. Rounding causes errors; if ratios are near simple fractions (e.g., 1.5), multiply by 2 to reach whole numbers.

Key notes

Important points to keep in mind

Always convert mass to moles before using mole ratios.

Use coefficients from the balanced equation as direct mole ratios.

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

When ratios give fractions like 1.5, multiply all by 2 to obtain whole numbers.

Keep track of significant figures; round only at the final step.

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

Check answers by confirming conservation of mass where applicable.