Unit 7B Classification of Chemical Reactions: Mole and Mass Relationships

Unit 7B: Classification of Chemical Reactions: Mole and Mass Relationships

Learning Objectives

  • Review the definition of the mole and convert between the number of moles and the number of particles of a substance.
  • Use molar mass to convert between the mass and the number of particles of a substance.
  • Convert between mass, moles, and the number of particles (ions, atoms, formula units) of a substance.
  • Determine molar ratios of reactants and products using balanced chemical equations.
  • Calculate the number of moles of products or reactants required for a chemical reaction to occur using balanced chemical equations.
  • Using mole ratios, calculate the mass of product that can be formed from a given mass of reactant and vice versa.
  • Calculate the theoretical yield and percent yield for a reaction.

Concepts to Review

  • Problem Solving: Unit Conversions and Estimating Answers – Unit 1
  • Molecular Formulas and Formula Units – Unit 4 Pts 1&2
  • Balancing Chemical Equations – Unit 7 Pt1

The Mole and Avogadro’s Number

  • The mole: A counting unit used for very small particles such as atoms, ions, molecules, and formula units.
  • Learning Objective: Review the definition of the mole and convert between the number of moles and the number of particles of a substance.

Definition of Mole

  • A counting term used to state a specific number of items. For example, terms like dozen, case, gross, and ream serve similar purposes in counting.

Avogadro's Number

  • The mole, defined as Avogadro's number, contains 6.02 imes 10^{23} items.
    • Used for counting small particles such as atoms, molecules, and ions.
    • 1 mole = 6.02 imes 10^{23} items
    • Avogadro's number is named after Amedeo Avogadro (1776–1856), an Italian physicist.

Mole Content

  • 1 mole of an element = 6.02 imes 10^{23} atoms of that element.
    • Example: 1 mole of carbon = 6.02 imes 10^{23} atoms of carbon.
    • Example: 1 mole of sulfur = 6.02 imes 10^{23} atoms of sulfur.

Conversion Factor - Avogadro's Number

  • Used to convert moles of a substance to particles.
  • Equality: 1 mole = 6.02 imes 10^{23} particles.

Study Check: Guide to Calculating Atoms or Molecules

  1. How many CO_2 molecules are in 0.500 mole of CO_2?
  2. The number of atoms in 2.0 moles of Al is: ____.
  3. The number of moles of S in 1.8 imes 10^{24} atoms of S is: ____.

Gram-Mole Conversions

  • Molar mass: The mass of 1 mole of substance = Mass of 6.022 imes 10^{23} molecules = Molecular weight in grams.
  • Learning Objective: Use molar mass to convert between the mass and the number of particles of a substance.
    • Convert between mass, moles, and the number of particles (ions, atoms, formula units).

Molar Mass Units

  • Measured in g/mol.
  • Examples:
    • The molar mass of ethylene (C2H4) is 28.0 g/mol.
    • The molar mass of lithium sulfide (Li2S) is 45.95 g/mol.
    • Atomic weight of 2 Li: 2 imes 6.941 ext{ amu} = 13.882 ext{ amu}
    • Atomic weight of 1 S: 32.07 amu.
    • Formula Weight (FW) of Li2S = 45.95 amu.
    • Molar mass of a compound: Numerically equal to molecular or formula weight, expressed as g/mol.

Calculations Using Molar Mass with Avogadro’s Number

  • Converting between grams and moles:
    • Molar mass Equality: 1 mole = g of pure substance.
    • Conversion Factors: For converting to number of particles, use Avogadro's number.

Practice Problem: Guide to Calculating Particles from Mass or Mass from Particles

  • A tiny pencil mark visible to the naked eye contains about 3 imes 10^{17} atoms of carbon. What is the mass of this pencil mark in grams?
  • Pseudoephedrine hydrochloride (C_{10}H_{16}ClNO): How many molecules are in a tablet that contains a dose of 30.0 mg?

In-Class Question

  • What is the mass in grams of 3.2 imes 10^{22} molecules of water?
    a. 0.0029 g
    b. 339 g
    c. 0.90 g
    d. 0.96 g

Moles of Elements in a Formula

  • Example: Aspirin, C_9H_8O_4, has the following:
    • 9 atoms of Carbon (C) = 9 moles of C.
    • 8 atoms of Hydrogen (H) = 8 moles of H.
    • 4 atoms of Oxygen (O) = 4 moles of O.
  • Subscripts: Directly relate the number of atoms in one molecule and can be used to write conversion factors for moles of each element in 1 mole of a compound.

Study Check: Guide to Calculating Atoms of Element in a Compound

  • How many atoms of O are in 0.150 mole of aspirin, C_9H_8O_4?

Practice Problem

  • Maximum dose of sodium hydrogen phosphate (Na_2HPO_4, MW = 142.0 amu) for use as a laxative is 3.8 g. How many moles of Na^+ ions and total moles of ions are in this dose?

In-Class Question

  • For Ca_5(PO_4)_3OH (hydroxyapatite), how many moles of Ca^{2+} ions are present in 1 mol of hydroxyapatite?
    a) 5 moles of Ca^{2+} ions
    b) 1 mole of Ca^{2+} ions
    c) 1.39 imes 10^{24} Ca^{2+} ions
    d) 6.00 imes 10^{23} Ca^{2+} ions

Mole Relationships and Chemical Equations

  • In chemical reactions, the unit specifying the relationship between reactants and products is the mole.
  • Learning Objective: Determine molar ratios of reactants and products using balanced chemical equations.

Law of Conservation of Mass

  • Indicates that in an ordinary chemical reaction:
    • Matter cannot be created or destroyed.
    • No change in total mass occurs.
    • Mass of products equals mass of reactants.

Coefficients in a Chemical Equation

  • Tell how many molecules (or formula units), thus how many moles, of each reactant are needed, and how many of each product are formed.

Information from a Balanced Equation

  • Example: In the equation 2Ag(s) + S(s) ightarrow Ag_2S(s):
    • 2 moles of Ag = 2(6.02 imes 10^{23}) Ag atoms
      defining particle counts, molar relationships, and corresponding grams.

Mole–Mole Factors from an Equation

  • In the equation 2Fe(s) + 3S(s) ightarrow Fe_2S_3(s):
    • Interpret as: 2 moles of Fe react with 3 moles of S to yield 1 mole of Fe_2S_3.

Molar Ratios Example

  • Example equation: N_2 + 3H_2
    ightarrow 2NH_3 can be interpreted as 1 mole of N2 reacts with 3 moles of H2.