Stoichiometry and the Mole Concept

Grade 10 · Science · Study Guide

Master the mole concept and stoichiometric calculations to relate quantities of reactants and products in chemical reactions.

Stoichiometry is the branch of chemistry that deals with the quantitative relationships between reactants and products in chemical reactions. At its core is the mole concept, a counting unit that allows chemists to work with the enormous numbers of atoms and molecules involved in reactions.

One mole is defined as exactly 6.022 × 10²³ particles (Avogadro's number). This number was chosen so that one mole of any element has a mass in grams equal to its atomic mass. For example, one mole of carbon-12 has a mass of exactly 12 grams.

Molar mass is the mass of one mole of a substance, expressed in grams per mole (g/mol). To find the molar mass of a compound, sum the atomic masses of all atoms in the formula. Water (H₂O) has a molar mass of 2(1.008) + 16.00 = 18.016 g/mol.

Stoichiometric calculations use balanced chemical equations as recipes. The coefficients in a balanced equation represent mole ratios. For the reaction 2H₂ + O₂ → 2H₂O, the ratio of H₂ to O₂ to H₂O is 2:1:2. These ratios let us calculate how much product forms from a given amount of reactant, or how much of one reactant is needed to fully react with another.

Key terms

Mole
The SI unit for amount of substance; equal to 6.022 × 10²³ particles (Avogadro's number)
Molar mass
The mass of one mole of a substance, numerically equal to the substance's atomic or molecular mass in grams per mole
Stoichiometry
The calculation of quantities of reactants and products in chemical reactions based on balanced equations
Mole ratio
The ratio of moles of one substance to moles of another in a balanced chemical equation
Limiting reactant
The reactant that is completely consumed first in a reaction, determining the maximum amount of product formed
Percent yield
The ratio of actual yield to theoretical yield expressed as a percentage, indicating reaction efficiency

Flashcards

Avogadro's number
6.022 × 10²³; the number of particles in exactly one mole of a substance
Molar mass calculation
Sum of (atomic mass × number of atoms) for every element in the formula
Moles from mass formula
n = m ÷ M, where n = moles, m = mass in grams, M = molar mass in g/mol
Theoretical yield
The maximum amount of product that could form based on stoichiometric calculations from the limiting reactant
Mole ratio
The coefficients in a balanced chemical equation that relate moles of reactants and products to each other
Excess reactant
The reactant that is not completely consumed in a reaction; some remains after the limiting reactant is used up
Percent yield formula
% yield = (actual yield ÷ theoretical yield) × 100%
Empirical formula
The simplest whole-number ratio of atoms of each element in a compound

Questions

  1. How many moles are in 44.0 grams of carbon dioxide (CO₂)? (Molar mass of CO₂ = 44.01 g/mol)

    1. 0.50 mol
    2. 1.00 mol
    3. 2.00 mol
    4. 44.0 mol
    Show answer

    Answer: 1.00 mol

    Moles = mass ÷ molar mass = 44.0 g ÷ 44.01 g/mol ≈ 1.00 mol

  2. In the balanced equation 2H₂ + O₂ → 2H₂O, how many moles of water are produced when 4 moles of H₂ react completely?

    1. 2 moles
    2. 4 moles
    3. 6 moles
    4. 8 moles
    Show answer

    Answer: 4 moles

    The mole ratio of H₂ to H₂O is 2:2 (or 1:1). So 4 moles of H₂ produce 4 moles of H₂O

  3. Avogadro's number is approximately 6.022 × 10²³ and represents the number of particles in one mole of any substance.

    Show answer

    Answer: True

    Avogadro's number (6.022 × 10²³) is the experimentally determined number of particles in one mole of any substance

  4. The reactant that is completely used up first in a chemical reaction and limits the amount of product formed is called the ________ reactant.

    Show answer

    Answer: limiting

    The limiting reactant is consumed first and determines the theoretical yield of the reaction

  5. What is the molar mass of glucose (C₆H₁₂O₆)? (C=12.01, H=1.008, O=16.00 g/mol)

    1. 30.03 g/mol
    2. 180.16 g/mol
    3. 168.15 g/mol
    4. 96.08 g/mol
    Show answer

    Answer: 180.16 g/mol

    Molar mass = 6(12.01) + 12(1.008) + 6(16.00) = 72.06 + 12.096 + 96.00 = 180.156 ≈ 180.16 g/mol

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