뒤로Chapter 6: Chemical Reactions—Mole and Mass Relationships
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Chapter 6: Chemical Reactions—Mole and Mass Relationships
6.1 The Mole and Avogadro’s Number
This section introduces the concept of the mole, Avogadro’s number, and their importance in quantifying chemical substances. Understanding these concepts is fundamental for relating mass, number of particles, and chemical reactions.
Atomic Weight: The average mass of an element’s atoms, typically expressed in atomic mass units (amu).
Molecular Weight (MW): The sum of the atomic weights of all atoms in a molecule. For ionic compounds, this is called formula weight.
The Mole: A mole is the amount of a substance whose mass in grams is numerically equal to its molecular or formula weight.
Molar Mass: The mass in grams of 1 mole of a substance; numerically equal to the molecular weight.
Avogadro’s Number: One mole contains molecules or formula units.
Sample Comparison: 1 mole of sulfur, copper, mercury, and helium each contains the same number of particles, but their masses differ due to different molar masses.
Example: Calculating the molar mass of pseudoephedrine hydrochloride and determining the number of molecules in a 30.0 mg tablet.
Additional info: Avogadro’s number is a fundamental constant used to relate the number of particles to the amount of substance in moles.
6.2 Gram-Mole Conversions
Gram-mole conversions are essential for translating between the mass of a substance and the number of moles present. The molar mass serves as the conversion factor.
Molar Mass: The mass of 1 mole of a substance, equal to its molecular weight in grams.
Conversion Factor: Molar mass is used to convert between grams and moles.
Example: Determining the mass of carbon in a pencil mark given the number of atoms, using the atomic weight and Avogadro’s number.
Additional info: The conversion process involves unit analysis to ensure correct cancellation and result.
6.3 Mole Relationships and Chemical Equations
Chemical equations represent the relationships between reactants and products in terms of moles. The coefficients in balanced equations indicate the ratios needed for calculations.
Mole Ratios: Derived from the coefficients in a balanced chemical equation; used as conversion factors.
Balanced Equations: Essential for determining the correct proportions of reactants and products.
Example: Calculating the mole ratios in the rusting of iron and determining the moles of iron oxide formed from a given amount of iron.
6.4 Mass Relationships and Chemical Equations
Mass relationships in chemical reactions require converting between mass and moles using molar masses and mole ratios. This section outlines the steps for such calculations.
Types of Conversions:
Mole-to-mole: Use mole ratios from the equation.
Mole-to-mass and mass-to-mole: Use molar mass as the conversion factor.
Mass-to-mass: Requires sequential conversions (mass to moles, mole ratio, moles to mass).
Calculation Steps:
Write the balanced chemical equation.
Choose molar masses and mole ratios for conversion.
Set up the factor-label expression.
Calculate and check the answer against an estimate.
Example: Calculating the mass of nitric acid produced from a given amount of nitrogen dioxide, using mole ratios and molar mass.
6.5 Limiting Reagent and Percent Yield
Not all reactions proceed with all reactants being fully consumed. The limiting reagent determines the maximum amount of product formed, and percent yield measures the efficiency of a reaction.
Limiting Reagent: The reactant that is completely consumed first, limiting the amount of product formed.
Theoretical Yield: The calculated maximum amount of product possible from the limiting reagent.
Actual Yield: The measured amount of product obtained from the reaction.
Percent Yield: The ratio of actual yield to theoretical yield, expressed as a percentage.
Example: Calculating percent yield for the combustion of acetylene, and determining the theoretical yield of boron from boric oxide and magnesium, including identification of the limiting reagent.
Worked Examples Summary Table
The following table summarizes the types of calculations and conversion factors used in the worked examples throughout the chapter:
Example | Given | Find | Conversion Factors |
|---|---|---|---|
Pseudoephedrine hydrochloride | Mass (mg) | Molecules | Molar mass, Avogadro’s number |
Pencil mark (carbon atoms) | Number of atoms | Mass (g) | Atomic weight, Avogadro’s number |
Ibuprofen in bottle | Moles | Mass (g) | Molar mass |
Sodium hydrogen phosphate | Mass (g) | Moles, ions | Molar mass, formula composition |
Rusting of iron | Moles of Fe | Moles of Fe oxide | Mole ratio from equation |
Nitrogen dioxide reaction | Moles of NO2 | Mass of HNO3 | Mole ratio, molar mass |
Calcium oxalate formation | Mass of product | Mass of reactant | Molar mass, mole ratio |
Acetylene combustion | Actual yield | Percent yield | Theoretical yield calculation |
Boron production | Masses of reactants | Theoretical yield | Molar mass, limiting reagent, mole ratio |
Ethylene to ethyl alcohol | Mass of ethylene | Actual yield | Molar mass, percent yield |
Key Equations
Mole Calculation:
Mass Calculation:
Number of Particles:
Percent Yield:
Summary
This chapter provides the foundational tools for quantifying chemical reactions, including the use of the mole, Avogadro’s number, molar mass, and balanced equations. It covers conversions between mass, moles, and number of particles, as well as the calculation of limiting reagents and percent yield. Mastery of these concepts is essential for problem-solving in general, organic, and biological chemistry.
